Check out Martin Van Creveld's article on Moshe Dayan in Vietnam and the parallels to Iraq today:

In other words, he who fights against the weak – and the rag-tag Iraqi militias are very weak indeed – and loses, loses. He who fights against the weak and wins also loses. To kill an opponent who is much weaker than yourself is unnecessary and therefore cruel; to let that opponent kill you is unnecessary and therefore foolish. As Vietnam and countless other cases prove, no armed force however rich, however powerful, however, advanced, and however well motivated is immune to this dilemma. The end result is always disintegration and defeat; if U.S troops in Iraq have not yet started fragging their officers, the suicide rate among them is already exceptionally high. That is why the present adventure will almost certainly end as the previous one did. Namely, with the last US troops fleeing the country while hanging on to their helicopters’ skids."

[hat tip to Fred.]