Are the American people war weary? Yes, to some degree. Could there be a worse prescription for American foreign policy than giving in to popular war weariness? No.
It was (well-deserved) war weariness after World War II that led to a precipitous drawdown in Europe that in turn helped make possible Stalin's subjugation of Eastern Europe. It was understandable war weariness after Vietnam that produced the shameful abandonment of Vietnam and Cambodia and the subsequent disastrous weakness of the Carter administration. It was (somewhat inexplicable) war weariness after the Cold War that led to a conviction in the 1990s, as Haley Barbour put it just last week, trying to accommodate the Paulistas, that "We're not the policeman of the world."
And thus we had the failure to finish the job in Iraq in 1991, the retreat under fire from Somalia in late 1993, inaction in Rwanda in 1994, years of dithering before confronting Milosevic in the Balkans, passivity in the face of the Taliban takeover of Afghanistan, and weak responses to al Qaeda's attacks on U.S. embassies in 1998 and the USS Cole in 2000. That decade of not policing the world ended with 9/11.
I'm sorry, how is Rwanda a failure of American foreign policy? What vital interest did we have there that required our risking of American lives? Or how about Somalia? Or anything in the Balkans? I realize we got involved with both but I certainly don't think we should have been even more involved. Imagine nation building in Somalia? Are we supposed to be invading every country where there is a problem? Would he have suggested that we intervene in Eastern Europe after World War II to stop the Stalinization of Poland and Czechoslovakia? I'm from the Soviet Union and I hate the Communists but don't think that such a move would have been a good idea. How many millions would have died in such a conflagration? I think someone should ask Bill Kristol "in what cases do you not think American force should be used." I'm not sure he would be able to think of any. If we had followed Bill Kristol's advice, our nation would be far poorer, far weaker and thousands or even millions of Americans would have lost their lives needlessly. Reagan showed that you don't actually have to use American might to get what you want. In 8 years, he only got us involved in a limited operation in Grenada, a one day bombing of Libya and a peacekeeping mission in Lebanon. Of all three, the only one that was a disaster was Lebanon, which is exactly the type of operation that Kristol favors.