Costs.

Via Atrios, this quote from Zbigniew Brzezinski, Carter's NSA:

The war has proven to be prohibitively costly. American leadership, in all of its dimensions, has been damaged. American morality has been stained – in Abu Ghraib and Guantanamo. American legitimacy has been undermined – by unilateral decisions. American credibility – particularly the case for the war, has been shattered. Leadership depends on morality, legitimacy, credibility. The economic costs of the war are escalating into hundreds of billions of dollars. More importantly, American casualties are in the thousands, with more than tens of thousands maimed. We are not even counting Iraqi casualties; we prefer not to know what they are.

Though I must admit, I first momentarily thought this was Kissinger talking. Then I realized 'Wait, he's a fucking war criminal.' Which is also how history will be referring to this Administration en masse.