I'm afraid that your "pre-1940s" view of the US is either rose tinted or just plain incorrect. While Teddy Roosevelt spoke of the need to speak softly and carry a big stick, the foreign policy of the United States has been largely the opposite of the isolationist position that many people seem to think is our norm. There's a reason the USMC's Battle Hymn starts with "from the Hallf of Montezuma to the shores of Tripoli." We've invaded both Canada and Mexico in our history, and (prior to the 1940 date you remember fondly) had been at war on every continent save Australia and Antartica. We took the vast majority of our nation away from the people who already lived there. I'm reasonably certain that every single US extraterritorial possession (i.e. Guam, etc) was in our possession prior to the second world war, except for a bunch of tiny atolls in the Pacific we built bases on during the war and maybe kept afterward.
I personally think that, on the whole, we've been a stronger force for "good" (however you want to define that) than "evil" but I do have my biases.