First self-interest. The German and Japanese goal was to conquer everybody. If they had conquered Europe and the Far East would they have stopped. It is true that in 1941 the oceans posed a greater barrier than they do today. But I still think the US would have been at risk.
Second there is the factor of solidarity with other civilzed countries like England. Would you really have us stand by while England went down (as it nearly did)? I would not. I subscribe to the notion of avoiding foreign adventures but enough is enough. Hitler posed a great danger to free peoples around the world and to me that was perhaps America's finest hour.
As to Japan I'll have to confess ignorance and let you fight it out with others. I just don't know much about this.
About Reagan. His failure to exercise the veto was kind of a quid pro quo. That is Democrats I'll let you ram through your domestic spending if you'll give me a free hand with defense. A good bargain IMHO. I mean what is the monetary value of defeating the USSR? Almost incalculable I think.
I agree that I wish Bush would spend less. But you gotta do what you gotta do. The priority is to win the war wiith Islam not the budget.