In my opinion, to answer my own question, I think think things would turn out either the same or perhaps better. In all cases, more money would lilely be poured into technology to make up for the lack of people so we could kill more with less.
World War I: We barely did anything, turns out the same. Psycological effects of us entering on the Germans are still present.
World War II: The Pacific theater, the one section where we did the most work, would probably be different but would have the same conclusion. I suspect, because of less people, there would only be a campagn through Micronesia, forgetting about the Solomons or the Philippines. More emphasis on starving the Japanese to death while a possible invasion is out of the question. Europe turns out the same, but more Russians die.
Korean War: The UN supplies more people. Problem solved. Same.
Vietnam War: We actually win it! With no conscription, people won't protest. It will be similar to the Iraq War, people don't like it but because they are not affected, won't bother to try to get us out. With an indifferent populace, Vietnam could be won.
I was talking about post-1900.