Why wasn't the US involved in World War 2?
The US is not obligated to take sides in every war. If Germany and France want to go to war with each other, that is not necessarily the problem of the US. Remember that wars generally kill lots of people and cost lots of money; if you can avoid them it is a good idea. Why would anyone want to die in a war if they don't have to? Of course, once the US itself was attacked (by the Japanese, at Pearl Harbor) then the reasons for going to war became much more compelling.