Burkeman1 is right that we were bamboozled into WWI by Wilson. He was the first liberal president to inflict serious damage on this nation. Washington warned us not to get involved in the European wars. Left to their own devices, the Europeans eventually would have bled themselves out in WWI. Our boys did not have the proper equipment in most instances and were slaughtered. We did manage to save Paris (in retrospect maybe we shouldn't have).
In WWII, the same thing would have happened. The Nazis and the Commies would have continued to smash each other until there was nothing left-which is a good thing. IMHO, our involvement should have been left to providing air cover to England at most. Some would have a knee jerk reaction saying that we helped to end the holocaust. While a nice thought, I don't think thats the case. FDR couldn't have cared less-in fact he turned a boat load of escaping Jews back to Germany early on in the war. Second, Stalin wound up killing just as many Jews as Hitler if not more. He is not villified for it in the history books because liberals don't want to say anything bad about communism which is just as evil, if not more so, than fascism.