oooh oooh, WWII stuff, here's my favorite -
the US kept out of the European fighting mainly because, as said, they figured it wasn't their problem. Europe was duking it out again, so let them deal, no need to get involved in another mess there, especially after WWI. Same with the Japanese rampaging through Asia. After the Germans invaded the USSR, the US stood even further back as they hoped that the fascist dictators Hitler and Stalin would just take care of each other. The US sent a lot of goods and aid to the UK via Atlantic sealanes, but millions of tons of it ended up at the bottom of the ocean courtesy of the German u-boat wolfpacks. Of course, profiteers here also sold tons of goods kinda quietly to the fascists, as well, but 'officially' the US disapproved of them (even though there was a quiet approval of Hitler's campaign against the Jews an Gypsies and such since they were considered as undesirables by many here, as well). The conflict, although terrible, was not an American problem. For the moment...
When the petroleum situation started to get dire in Japan, things started to get heated (sound familiar?). They looked to attack the US, cripple her Pacific fleet, and then have free reign over oil supplies in the Pacific and over whichever territories they wanted. SO they attacked at Pearl Harbor and annihilated most of the US Navy's Pacific fleet (except the carriers, which ended up biting them in the butt later at Midway). At THAT point, the US declared war on Japan. Then, Nazi Germany declared war on the US in accordance with the Rome-Berlin-Tokyo Axis AND the fact that the Japanese had hinted that they'd open up a second front against the Russians, which the Germans desperately needed at this point. SO now the US was fully involved in WWII. Course by the time they setup shop in the (hammered) UK and started air-raids over occupied France and the German homeland, it took quite some time, which only figures as it was a HUGE undertaking. 1942 saw the earliest American action against the Germans and the Japanese. Don't forget Pearl Harbor was in December 1941.
For the record, France did help quite a bit against the Germans. Yeah they surrendered pretty quickly but the Germans had taken Paris in a matter of days. They'd trampled through Holland and Belgium and, to most everyone's surprise, through the Ardenne forest. The French were left going "WTF!?!?" (in Francaise, of course) and everyone else was kinda like "wow, that was...impressive." The Free French, though, were an integral part of the Allied war effort. Without their resistance efforts in Occupied France, the war would likely have dragged on much longer. Of course, in contrast to all the things the Free French did, there's also Vichy France.
So really, the French aren't quite as spineless as some would have people believe. Remember that the American Revolution would also have taken a much different path if the French hadn't joined up with the Americans to help fight the British, but how quickly (and conveniently) people forget, heh.
Not taking sides or anything here, just expanding on the WWII talk and kinda playing devil's advocate