The public's opinion of past wars improves as a new war approaches. Thus, after Vietnam most people thought the war was a mistake and this held true for decades until the beginning of the Iraq war when the opinion of war in Vietnam suddenly improved! Even more dramatically, a majority of people thought that World War I was a mistake until World War II approached when the percentage thinking it was a good war doubled.
Irrational as this is, it makes a certain sense. Here's my theory: In the face of something as awful as war, it's comforting to think it will actually mean something worthwhile. But people judge by past performance, and if America has fought wars it shouldn't have in the past, that raises an unpleasant possibility- maybe this war isn't worth fighting, either. Maybe the suffering and death won't mean anything worthwhile, after all.
That's an unpleasant thought. Thus, a good way to maintain the consoling belief that a current or upcoming war and its attendant horror is worthwhile and meaningful is to convince yourself that America's past track record in picking wars is a good one, even if you didn't believe that previously. People are pretty good at believing what they want to believe.This desire to get meaning out of senseless tragedy has consequences beyond bad historical interpretation. In my own experiences, I've heard a number of people argue for staying in Iraq on grounds that boil down to (and are often explicitly expressed as), "If we quit now, the Americans who died there will have died in vain. Therefore, we must keep fighting to make their deaths worthwhile." The cost of war thus becomes its own justification: In order to give meaning to the thousands of deaths the war has already brought, even more lives must be fed to the fire.