They say, well, life is so complicated, the Balkans are so complicated, there's nothing you can do there, they just kill each other all the time. First of all, you find out that it's not true, that much of what is presented to you as history, whether by ideologues and publicists, or even by the press in good faith, turns out to be such an encapsulation of history that it's not the real history. Historical study should awaken you to complexity, but not to a point where you can't act. A moral human being needs to appreciate that knowledge is partial and that you could be wrong in acting yourself or in recommending action to others. The crucial moral test is acting, knowing that you could be wrong.
Some people act on the assumption they're always right, and they are monsters.
Other people are paralyzed by the awareness of the complexity of things, and the great test is whether you can act on the basis of your best sense of what the case is. But you must keep thinking after you have acted. In the case of Vietnam, we committed ourselves, and then even when people like McNamara believed that they were wrong, they couldn't or wouldn't struggle for withdrawal. It's partially psychological. On the basis of your original conviction, you have sacrificed people.