WASHINGTON -- Without warning in the dead of night of Jan. 3, on a dirt road in a remote region of Pakistan, two missiles slammed into a double-cab pickup truck and blew it to smithereens along with the six men inside. It's safe to say the victims never heard the U.S. drone circling far overhead.

One of the dead was a known bad guy, Mullah Nazir, a Taliban warlord who boasted of his ties with al Qaeda and recently banned polio vaccinations for local children. The other men killed were said to be lower-ranking Taliban commanders.

A clean kill, demonstrating the increasing ability of the United States to identify, track, target and eliminate dangerous threats to American security? Perhaps. A majority of Americans think so -- by 59 to 18, Americans approve of using drones to kill high-level terrorist suspects overseas, according to a new HuffPost/YouGov poll. But a growing number of military and civilian experts in war and law say that President Barack Obama's drone war is counterproductive and unsustainable, perhaps violating the Constitution as well.

That puts the expanding American fleet of 375 armed drones -- which gives the White House a seductively simple, inexpensive, safe (for Americans) and easily hidden way to wage war -- squarely on top of Obama's agenda when he takes up his second term Jan. 21. Complicating a potential reform of the drone program: Its chief architect and apologist, White House terrorism adviser John Brennan, is Obama's pick to run the CIA -- which operates in secret many of the drone strikes.

"Our criteria for using [drones] is very tight and very strict," Obama insisted in August. In an interview with CNN, Obama explained that any proposed strike has to comply with U.S. and international law, and the target must be a real threat who cannot be captured.

"And we have got to make sure that in whatever operations we conduct we are very careful about avoiding civilian casualties. And, in fact," the president said, "there are a whole bunch of situations where we will not engage in operations if we think there's gonna be civilian casualties involved."

On the morning of March 17, 2011, more than three dozen village elders and local government leaders gathered in an open-air bus depot in the town of Datta Khel, in North Waziristan, Pakistan. Under discussion: how to avoid being drawn into the insurgency raging there and across the border in Afghanistan. At about 10:45 a.m., a drone hovering overhead fired a supersonic missile into the gathering. One man remembers hearing a slight hissing noise before the blast threw him, unconscious, several yards away. An immediate second strike killed many of the wounded.

What happened in Datta Khel has been exhaustively documented, at some risk, in on-site investigations by The New York Times, the Associated Press, The Guardian, and the independent Bureau of Investigative Reporting, among other organizations. The results were verified and compiled in a report by law-school researchers from Stanford and New York universities.

U.S. officials insisted that all those killed were insurgents. But interviews with survivors and families of the dead, along with other eyewitnesses and medical authorities indicated that most if not all of the roughly 40 people killed were civilians. The Associated Press investigation concluded that four of the dead may have been affiliated with the Taliban.

That June, three months after the Datta Khel attack, Brennan boasted that in the drone attacks, "there hasn't been a single collateral death because of the exceptional proficiency, precision of the capabilities we've been able to develop."

But that one attack -- among 350 or more drone strikes Obama has authorized -- captures what critics contend is a lose-lose proposition for the United States as it confronts militant Islamist insurgencies and terrorist organizations across much of the world.

Almost all of the "targeted assassinations" of alleged terrorist leaders engaged in plots or operations against the United States are conducted by drone strikes. Others have been accomplished with AC-130 aerial gunships or by commandos.

Among the known, if not acknowledged, targets of drone strikes so far have been people in Afghanistan, Pakistan, Yemen and Somalia. Administration officials are said to be eyeing drone strikes against al Qaeda-linked insurgents in the mid-Sahara nation of Mali and in north Africa. Predator and Reaper drones can carry either Hellfire missiles with blast-fragmentation warheads, or the satellite-guided GBU-39 bomb, which can glide 60 miles and bears a carbon fiber composite warhead to reduce lethal shrapnel. The Air Force has bought 12,379 of these bombs from Boeing.

The accumulating problems caused by these killings concern even those who acknowledge them as a legitimate weapon of war. As U.S. troops are reduced in Afghanistan over the next two years, for instance, the United States may find drone strikes increasingly useful against insurgent sanctuaries across the border in Pakistan.

"The fact is, the U.S. might need to maintain and sustain this capability," Micah Zenko, a fellow at the Council on Foreign Relations told The Huffington Post. "But there needs to be significant restraints and much more transparency" both in the legal justification for the killings and in how the strikes are conducted.

Secrecy is a key concern. The administration has never explained why drone strikes -- and not, say, infantry attacks in Afghanistan -- must be secret. Yet with such a highly classified program, there is no effective independent confirmation of the administration's assertions that the strikes comply with domestic and international law. Strike packages are assembled by the CIA and the military's Joint Special Operations Command, whose role has grown from its initial involvement managing drone strikes in Afghanistan and Iraq.

The material, including identification of the targets, justification for the strikes, estimates of potential civilian casualties and assessments of the impact on the local community, are vetted by lawyers and officials at the Pentagon and White House -- in secret.

Obama has ducked questions about whether or not he personally reviews each strike package.

The legal basis for drone strikes is said to reside in a June 2010 memo from the White House Office of Legal Counsel used to justify killing Anwar al-Awlaki, a U.S. citizen and extremist firebrand, in Yemen in September 2011. That memo remains secret after New York District Court Judge Colleen McMahon ruled in frustration on Jan. 2 that the law shields those White House decisions. That effectively allows "the executive branch of our government to proclaim as perfectly lawful certain actions that seem on their face incompatible with our Constitution and laws, while keeping the reasons for its conclusions a secret," she wrote.

Nor is there much congressional oversight of the drone strikes. After being blocked for years, the Senate Intelligence Committee finally won permission in 2009 to send a delegation to the CIA once a month to peek at strike videos and scan some of the intelligence justifying the strikes, a committee official confirmed. But congressional committees charged with oversight of the armed services and foreign relations have never managed to hold even closed-door classified briefings on the drone strikes.

"Assertions by Obama administration officials as well as by scholars, that these operations comply with international standards are undermined by the total absence of any forms of credible transparency or verifiable accountability," concludes Philip Alston, NYU professor of law and the former United Nations adviser on extrajudicial, summary or arbitrary executions.

A second deep concern is the civilian death toll from drone strikes. In the enormous violence unleashed in war, civilians always suffer. After more than a decade of war, military targeters have gotten pretty good at designing attacks to minimize or eliminate civilian casualties. But not perfect. Drone operators can watch a proposed target for days or weeks to establish patterns of life, augmented by spies and cultural experts. Targeting software like Bugsplat, which models the effective kill zone and collateral injury and damage of a proposed strike, can help. "You use the full range of intelligence," said Army Brig Gen. Rich Gross, who as legal counsel to the Joint Chiefs of Staff reviews strike execution orders.

Counting the number of civilians killed in these strikes is notoriously difficult, given that strikes usually take place in remote areas that are often hostile to Westerners. The most careful accounting is generally considered to be that by the Bureau of Investigative Journalism, which said between 558 and 1,119 civilians have been killed in strikes in Pakistan, Yemen and Somalia.

In any attack, Gross told The Huffington Post, "there's a real science involved with what type of weapon system will be dropped and the numbers of people you would expect in that culture at that time of day." The art, he said, is judging whether the military benefit of a strike outweighs the projected loss of civilian life, as required by international law.

The recent use of "signature strikes" or "crowd killings," which are said to target a group of unnamed and unidentified suspects, appears to violate international law even more egregiously.

There is also the issue of blowback. It doesn't take a detailed military analysis to recognize that having drones constantly overhead promising instant death isn't popular in non-battlefield strike zones like Pakistan, Yemen and Somalia. Evidence gathered by reporters and investigators in North Waziristan and other sites of drone strikes is that the anger, fear and resentment the strikes leave behind among civilians seems to outweigh any potential military benefit. Such devastating strikes, which kill with no warning, "are hated on a visceral level," retired general and Afghan war commander Stanley McChrystal said recently. "The resentment created by American use of unmanned strikes," he added, "is much greater than the average American appreciates."

"The argument that several folks have raised is that when you kill a terrorist, even if you kill no women and children, no combatants, you're still gonna enrage the population, depending on how it's done," said Gross. "That's a consideration that policymakers always have to struggle with."

Two other problems arise with the drone program. One is copycats. Inevitably, weapons technology spreads, and already other countries are racing to expand their drone fleets and arm them with weapons. It's also likely that some will not be as fussy as the U.S. government said it is in following international law. Iran has already used surveillance drones. Imagine the havoc if it threatened to launch clouds of armed drones toward the Persian Gulf oilfields.

For that reason, experts have urged the administration to work closely to control the export of drone technology, as the United States and others have done with nuclear weapons technology.

The other issue is whether or not drone killings really advance the so-called war on terror. The use of secretive drone strikes is justified as a way to "surgically" (to use Brennan's description) remove senior terrorist leaders intent on attacking the United States. The Jan. 3 strike which killed Mullah Nazir, for instance, ostensibly removed a senior battlefield commander.

But, according to the Long Wars Journal, Nazir was replaced within a day by Bahwal Khan, his top aide for the past 16 years, an Afghan fighter closely allied with al Qaeda and other terrorist groups in the region. As a result, "little will change," a U.S. intelligence official told the Journal. "It will be business as usual, we'll continue to have to take shots at al Qaeda leaders and others" in the area, the official said.

Given all these difficulties, said Zenko, "the trajectory of U.S. drone strike policies is unsustainable." On the military side, a global arms race to acquire and deploy armed drones could erode the U.S. freedom of action and threaten American friends and allies.

On the domestic side, the rising clamor of public opinion could either force the program deeper into the shadows, or impose unwieldy or counterproductive reforms from the outside -- for example a ban on all armed drone strikes.

Many security experts acknowledge that armed drones provide the United States a needed national security capability which should be retained -- and can be done so safely with a little more transparency and safeguards.

For example, Zenko suggests in a new policy paper that the president should move quickly to disclose more of the legal justification for the strikes, end the practice of crowd killings, authorize classified briefings to Congress, and, to increase accountability, assign the drone program to either the Defense Department's Joint Special Operations Command or the CIA, but not both.

Beyond that, he said, the United States might usefully set up an international protocol to limit the spread of drone technology, much as it has done with nuclear weapons technology.

"Reforming U.S. drone strike policies will be difficult and will require sustained high-level attention to balance transparency with the need to protect sensitive intelligence sources and methods," Zenko writes.

Over to you, Mr. President.

This article is part of a series produced by The Huffington Post that closely examines the most pressing challenges facing President Obama in his second term. To read other posts in the series, click here.

This story appears in Issue 33 of our weekly iPad magazine, Huffington, in the iTunes App store, available Friday, Jan. 25.

Also on HuffPost:

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  • Situation Room

    May 1, 2011: President Barack Obama and Vice President Joe Biden, along with members of the national security team, receive an update on the mission against Osama bin Laden in the Situation Room of the White House, May 1, 2011. Seated, from left, are: Brigadier General Marshall B. "Brad" Webb, Assistant Commanding General, Joint Special Operations Command; Deputy National Security Advisor Denis McDonough; Secretary of State Hillary Rodham Clinton; and Secretary of Defense Robert Gates. Standing, from left, are: Admiral Mike Mullen, Chairman of the Joint Chiefs of Staff; National Security Advisor Tom Donilon; Chief of Staff Bill Daley; Tony Blinken, National Security Advisor to the Vice President; Audrey Tomason Director for Counterterrorism; John Brennan, Assistant to the President for Homeland Security and Counterterrorism; and Director of National Intelligence James Clapper. Please note: a classified document seen in this photograph has been obscured. (Official White House Photo by Pete Souza)

  • Head Pat

    May 8, 2009: President Barack Obama bends over so the son of a White House staff member can pat his head during a family visit to the Oval Office May 8, 2009. The youngster wanted to see if the President's haircut felt like his own. (Official White House Photo by Pete Souza)

  • Fist-Bump

    Dec. 3, 2009: President Barack Obama fist-bumps custodian Lawrence Lipscomb in the Eisenhower Executive Office Building following the opening session of the White House Forum on Jobs and Economic Growth, Dec. 3, 2009. (Official White House Photo by Pete Souza)

  • Mohawk Baby

    July 4, 2012: President Barack Obama holds a baby while greeting guests during an Independence Day celebration on the South Lawn of the White House, July 4, 2012. (Official White House Photo by Pete Souza)

  • The President

    July 26, 2012: President Barack Obama holds a Cabinet meeting in the Cabinet Room of the White House, July 26, 2012. (Official White House Photo by Pete Souza)

  • Oval Office Chase

    July 9, 2012: President Barack Obama runs around his desk in the Oval Office with Sarah Froman, daughter of Nancy Goodman and Mike Froman, Deputy National Security Advisor for International Economics, July 9, 2012. (Official White House Photo by Pete Souza)

  • First Couple

    June 13, 2012: President Barack Obama and First Lady Michelle Obama wave goodbye to President Shimon Peres of Israel on the North Portico of the White House following the Presidential Medal of Freedom ceremony and dinner in his honor, June 13, 2012. (Official White House Photo by Pete Souza)

  • Holocaust Museum

    April 23, 2012: President Barack Obama tours the United States Holocaust Memorial Museum in Washington, D.C., with Elie Wiesel, Nobel Peace Prize laureate and Holocaust survivor, and Sara Bloomfield, museum director, April 23, 2012. (Official White House Photo by Pete Souza)

  • Golden Girl

    June 11, 2012: President Barack Obama talks with Betty White in the Oval Office, June 11, 2012. (Official White House Photo by Pete Souza)

  • Venus In Transit

    April 24, 2012: President Barack Obama stops to view the moon and Venus before boarding Marine One in Boulder, Colo., April 24, 2012. (Official White House Photo by Pete Souza)

  • 'Sweet Home Chicago'

    Feb. 21, 2012: President Obama joins in singing "Sweet Home Chicago" during the "In Performance at the White House: Red, White and Blues" concert in the East Room of the White House, Feb. 21, 2012. Participants include, from left: Troy "Trombone Shorty" Andrews, Jeff Beck, Derek Trucks, B.B. King, and Gary Clark, Jr. (Official White House Photo by Pete Souza)

  • The Redeemer

    March 20, 2011: "The Obama family was scheduled to tour the Christ the Redeemer statue in Rio before dinner one night. But when heavy fog rolled in, they canceled the visit. After dinner, the fog had dissipated somewhat so they decided to make the drive up the mountain. It was quite clear when they arrived and then the fog started to roll back in. I managed to capture this silhouette as they viewed the statute one last time just before departure." (Official White House Photo by Pete Souza)

  • Water Fight

    June 11, 2011: "The top photograph shows the President having a water gun fight with his daughter Sasha on her birthday weekend at Camp David. Unbeknownst to me, David Lienemann captured a similar photo of the Vice President on the very same day." (Official White House Photos by Pete Souza and David Lienemann)

  • Christmas Card

    Dec. 11, 2011: President Barack Obama, First Lady Michelle Obama, and their daughters, Malia, left, and Sasha, right, sit for a family portrait in the Oval Office, Dec. 11, 2011. (Official White House Photo by Pete Souza)

  • MLK Memorial

    Oct. 14, 2011: President Barack Obama tours the Martin Luther King, Jr. National Memorial in Washington, D.C., Oct. 14, 2011. (Official White House Photo by Pete Souza)

  • Balloon Man

    Oct. 11, 2011: "This photograph by Chuck Kennedy has to catch your eye. It shows Guinness Book of World Records holder John Cassidy performing a balloon act for First Lady Michelle Obama in the Diplomatic Reception Room following a Let's Move event." (Official White House Photo by Chuck Kennedy)

  • Mandela

    June 21, 2011: First Lady Michelle Obama meets with former President Nelson Mandela of South Africa at Mandela's home in Houghton, South Africa, June 21, 2011. (Official White House Photo by Samantha Appleton)

  • Joplin, Mo.

    May 29, 2011: President Barack Obama greets Hugh Hills, 85, in front of his home in Joplin, Mo., May 29, 2011. Hills hid in a closet during the tornado, which destroyed the second floor and half the first floor of his house. (Official White House Photo by Pete Souza)

  • Daycare

    June 9, 2011: President Barack Obama greets children at a day care facility adjacent to daughter Sasha's school in Bethesda, Md., following her 4th grade closing ceremony, June 9, 2011. (Official White House Photo by Pete Souza)

  • Tucson Shooting

    Jan. 10, 2011: President Barack Obama and First Lady Michelle Obama walk towards the White House after observing a moment of silence for the victims of the Arizona shooting, on the South Lawn, Jan. 10, 2011. White House staff joined the President and First Lady for the moment of silence.(Official White House Photo by Chuck Kennedy)

  • What's Up?

    March 7, 2011: President Barack Obama, Prime Minister Julia Gillard of Australia, and members of the Australian and American delegations look up at the presidential seal in the Oval Office ceiling following their bilateral meeting, March 7, 2011. (Official White House Photo by Pete Souza)

  • Inclement Weather

    May 31, 2010: "The skies opened up on Memorial Day outside of Chicago. When the lightning began, the Secret Service told the President that it was too dangerous to proceed. He took the stage by himself and informed the audience that his speech was canceled and that for everyone's safety, they should return to their busses. Later, he boarded a few of the busses to thank them for attending and apologized for not being able to speak." (Official White House Photo by Pete Souza)

  • Power Walk

    July 20, 2010: President Barack Obama and British Prime Minister David Cameron walk across the South Lawn of the White House, July 20, 2010. (Official White House Photo by Pete Souza)

  • Oil Spill

    May 28, 2010: President Barack Obama and Lafourche Parish President Charlotte Randolf, left, inspect a tar ball as they look at the effect the BP oil spill is having on Fourchon Beach in Port Fourchon, La., May 28, 2010. (Official White House Photo by Chuck Kennedy)

  • Finally

    March 21, 2010: President Barack Obama, Vice President Joe Biden, and senior staff, react in the Roosevelt Room of the White House, as the House passes the health care reform bill, March 21, 2010. (Official White House Photo by Pete Souza)

  • First Pitch

    March 31, 2010: President Barack Obama practices his pitching form with personal aide Reggie Love and Jake Levine in the Rose Garden of the White House, March 31, 2010. The President threw out the ceremonial first pitch on opening day of the baseball season prior to the game between the Washington Nationals and the Philadelphia Phillies.

  • 3D

    Feb. 1, 2009: "During a Super Bowl watching party in the White House theatre, the President and First Lady join their guests in watching one of the TV commercials in 3D." (Official White House photo by Pete Souza)

  • Bo

    March 15, 2009: "The Obama family was introduced to a prospective family dog at a secret greet on a Sunday. After spending about an hour with him, the family decided he was the one. Here, the dog ran alongside the President in an East Wing hallway. The dog returned to his trainer while the Obama's embarked on their first international trip. I had to keep these photos secret until a few weeks later, when the dog was brought 'home' to the White House and introduced to the world as Bo." (Official White House photo by Pete Souza)

  • Moment Of Reflection

    Jan. 20, 2009: "President-elect Barack Obama was about to walk out to take the oath of office. Backstage at the U.S. Capitol, he took one last look at his appearance in the mirror." (Official White House photo by Pete Souza)

  • Snowball Fight

    Dec. 19, 2009: "Snowball in hand, the President chases Chief of Staff Rahm Emanuel on the White House colonnade. To escape, Rahm ran through the Rose Garden, which unfortunately for him, was knee-deep in snow." (Official White House photo by Pete Souza)

  • Nobel

    Dec. 10, 2009: President Barack Obama looks at the Nobel Peace Prize medal for the first time at the Norwegian Nobel Institute in Oslo, Norway, Dec. 10, 2009. (Official White House Photo by Pete Souza)

  • Presidential Trio

    Jan. 16, 2010: "President Obama had called on the two former Presidents to help with the situation in Haiti. During their public remarks in the Rose Garden, President Clinton had said about President Bush, 'I've already figured out how I can get him to do some things that he didn't sign on for.' Later, back in the Oval, President Bush is jokingly asking President Clinton what were those things he had in mind." (Official White House photo by Pete Souza)

  • Wounded Warriors

    Oct. 8, 2009: President Barack Obama watches as members of the National Naval Medical Center's Marine Wounded Warrior basketball team play on the White House basketball court, Oct. 8, 2009. (Official White House Photo by Pete Souza)

  • Shoot The J

    Oct. 8, 2009: President Barack Obama takes a shot during a game with Cabinet secretaries and members of Congress on the White House basketball court, Oct. 8, 2009. (Official White House Photo by Pete Souza)

  • Grand Canyon

    Aug. 16, 2009: President Barack Obama looks at the Grand Canyon in Arizona on Aug. 16, 2009. (Official White House photo by Pete Souza)

  • Ted Kennedy

    April 21, 2009: President Barack Obama and Sen. Ted Kennedy walk down the South Lawn sidewalk at the White House April 21, 2009. (Official White House Photo by Pete Souza)

  • The Pope

    July 10, 2009: President Barack Obama and First Lady Michelle Obama meet with Pope Benedict XVI at the Vatican on July 10, 2009. (Official White House photo by Pete Souza)

  • Punch

    July 13, 2009: President Barack Obama feigns a punch while talking about health care reform with Nancy-Ann DeParle, Peter Orszag, Phil Schiliro and Larry Summers in the Outer Oval Office, July 13, 2009. (Official White House Photo by Pete Souza)

  • Teasing

    June 26, 2009: President Barack Obama jokingly reacts to news that staffer Nora Becker will be leaving to pursue a joint MD and PhD in healthcare economics, during the White House staff picnic on the South Lawn, June 26, 2009. (Official White House Photo by Pete Souza)

  • Nancy Reagan

    June 2, 2009: President Barack Obama and former First Lady Nancy Reagan walk side-by-side through Center Hall in the White House, June 2, 2009. To the left of Mrs. Reagan hangs her official White House portrait as First Lady. (Official White House Photo by Pete Souza)

  • Cairo Visit

    June 4, 2009: Secretary of State Hillary Clinton recounts a story to President Barack Obama, Senior Advisors David Axelrod and Valerie Jarrett, outside the Sultan Hassan Mosque in Cairo, Egypt, June 4, 2009. (Official White House Photo by Pete Souza)

  • The Sphinx

    June 4, 2009: President Barack Obama tours the Egypt's Great Sphinx of Giza (left) and the Pyramid of Khafre, June 4, 2009. (Official White House photo by Pete Souza)

  • Behind The Camera

    Feb. 18, 2009: President Barack Obama takes aim with a photographer's camera backstage prior to remarks about providing mortgage payment relief for responsible homeowners at Dobson High School, Mesa, Ariz., Feb. 18, 2009. (Official White House Photo by Pete Souza)

  • First Day

    Jan. 21, 2009: President Barack Obama walks into the Oval Office for his first full day in office, Jan. 21, 2009. His Personal Aide Reggie Love stands nearby. (Official White House photo by Pete Souza)