In their first debate, John McCain's constant refrain was that Barack Obama "didn't understand." Tonight, Obama grabbed that phrase and ran with it in the other direction, prefacing his remarks on how the economic downturn would affect our foreign policy goals by saying, "Senator McCain, in the last debate and again today suggested that I don't understand. It's true. There are some things I don't understand. I don't understand how we ended up invading a country that had nothing to do with 9/11." That launched a wide-ranging criticism of Bush's -- and by extension McCain's -- foreign policy. In the Biden-Palin debate, bringing up the cost of the Iraq War -- and especially mentioning the surpluses that Iraq enjoys, as we spend and spend -- resonated powerfully with an audience with the economy on the brain. It did so again tonight.