In my last decade of active military service, to include a stint as a student at the Army's School of Advanced Military Studies (SAMS, aka "the Jedi Knight School"), I heard this phrase bandied around on a fairly common basis. Al Qa'ida was called an "Existential Threat." North Korea was called an "Existential Threat." As was Iraq, Cuba, Iran, drugs, international terrorism, the tech bubble, the housing bubble and any popular music created after Led Zeppelin broke up (ok, I made up the last three; that doesn't make it untrue...some people remember when JFK was shot, I remember where I was when Zeppelin broke up).
The phrase, like my last comment above, has become not only overused, but somewhat of a joke in the inner circles of military planning. Could illegal drugs destroy the United States? Could a terrorist organization, no matter how well funded, organized or trained, eliminate the Republic from the face of the planet?
No. The reality is that there is only one nation on Earth, besides ourselves, who can physically destroy the United States -- Russia. I came up through the late Cold War U.S. Army, where posters in my basic training barracks at Fort Benning, Georgia (as an aside, the barracks were the finest quality construction of 1942, had maps of South Vietnam on the walls, and were still being used in 1983), were of life-sized Soviet soldiers and had the words "This is Your Enemy. Know Him." printed below. Thankfully, the world did not leap into that abyss and a 20 year old hillbilly from the Ozarks didn't find himself dressed in full chemical gear somewhere in central West Germany shooting anti-tank rockets at a 20 year old hillbilly from the Urals in a tank.
The end of the Cold War, however, did not put an end to the threat of nuclear war. While we pontificate about the "existential threat" of nuclear proliferation from Iran, et al, the simple fact remains that it would take hundreds of near simultaneous nuclear blasts to destroy the United States. Regardless of other issues that a nuclear conflict short of a massive exchange would cause (such as damage to the nation's electronic infrastructure from electromagnetic pulses from the blasts to even a small amount of nuclear soot being shoved into the atmosphere, and the concordant drop in global temperatures, etc.), no minor nuclear power could destroy the United States without a massive, nation-ending counterattack from the Americans. Only Russia possesses the number of weapons and delivery systems required to destroy the United States.
The new administration, with the economic crisis, health care and a myriad of other issues on their plate, needs to take on the one real "existential threat" to the Republic -- the sane, mutual reduction of nuclear weapons in the United States and the Russian Federation. Both sides can take action to reduce their massive stockpiles while still maintaining a sufficient deterrent to maintain deterrence.
First, both sides need to eliminate all land-based ICBMs. Quite simply, they do not need them and of all parts of the nuclear triad, they are the most destabilizing. Land-based missiles are vulnerable to first strike attacks, terrorist assaults aimed at obtaining a bomb or materials, and are the only real first strike nuclear weapons that both nations maintain. Set an example for North Korea and Iran by banning land-based ICBMs.
Second, allow for the increase in land-based manned strategic bombers. Doctor Strangelove not withstanding, land-based aircraft take time to arm, marshal and fly to their targets. Heaven forbid a nuclear crisis develops between the U.S. and the Russian Federation, it would give the leaders of both nations the ability to show their determination while giving both sides time to calm down. There is a substantial difference between 30 minutes from a missile launched in the Urals to a target in the U.S., and a bomber flying 15 hours from Louisiana to Moscow. Since we cannot immediately eliminate nuclear weapons from the planet, this is, I believe, a compromise that provides both sides with a viable option. And from a personal view, I am much more comfortable with a pilot in a bomber that can be recalled, than an ICBM that cannot.
Third, allow both sides to keep a secure deterrent -- ICBM submarines -- but with limited warhead and missile capacity. Both the U.S. and Russia would argue that their nuclear forces must be "redundant" and "survivable" -- both phrases from the Cold War full-scale war mentality. However, both sides could easily maintain a small, stealthy, and effective nuclear submarine force capable of acting even in a first strike/doomsday scenario. In effect, the submarines would guarantee both sides a final deterrent from a surprise attack -- the core fear of both American and Russian military leaders since 1941.
Lastly, the elimination of land-based missiles would be the first step in reducing both nuclear stockpiles to sane levels. Later, as air and sea launched weapons became obsolescent or needed to be replaced, both sides could agree to replace a reduced percentage over the years. If, for example, the first generation of nuclear weapons becomes outdated in 10 years, the US could replace 100 of these older systems with 50 newer ones. Then, as time passed, replace those 50 with 25 better weapons, and so on, while still maintaining a credible deterrent force.
The increase in lethality and accuracy of modern conventional weapons, and the lack of practical use for nuclear weapons since the end of World War II, means that military planners have no real necessity to depend upon these weapons. Of course, the reality of nuclear weapons is that they are less of a military tool than a political one. This means that it is the role of the political leadership, not the military, of both the U.S. and Russia to address the reduction of nuclear stockpiles.
Perhaps that is the real "existential threat" to both nations; unless their leaders act, today, to reduce nuclear weapons, our descendants will be cursed to live with the possibility, no matter how faint, that their fathers and grandfathers cursed them with a never-ending fear of the end of humanity in a flash.