Obama is right to be realistic and to abjure bombastic rhetoric. Moralism is expensive -- costly in blood and treasure. This is the new reality. The danger is that we will turn inward -- not isolationist, because that is impossible -- but financially exhausted and callously indifferent to the rest of the world.
The assumption that US unwillingness to use military force against ethnonationalistic actors led by loathsome ideologues -- the Taliban, for example -- comes with some fairly serious moral baggage. Realists aren't heartless: they're just circumspect about what will and what will not work. Cohen goes on to say that our withdrawal from Vietnam was "sickening to behold." Does he think it would have been morally right to stay and continue the war? A realistic view of the world would hold that foreign powers have limited capacity to change the morals of societies through force or coercion, and that great power military intervention rarely has any humanitarian benefit (but often causes tremendous harm). Without some sort of deal that will bring in moderate Pashtuns currently fighting under the Taliban flag, we could fight in Afghanistan quite literally forever, and a lot of people would get killed, and peace, democracy and development would not come. That would be moral? Why? Because our intentions were noble? The road to hell is paved with good intentions.
Still, Cohen's basic point is solid, and welcome. I never thought the day would come, but the Democratic Party is now the party of foreign policy realism. Long may it remain so.