It's not really clear why WWI was fought, other than "big power rivlary", whatever that means. Ten million people died for nothing. Today it is really hard to figure out what it was really all about. We know how the war started in excruciating detail (The Guns of August, Tuchman) but not why.

WWII is a little different. There really were snakes on the plane for that one. Fascism had to be stopped. But it wasn't always clear which side the US would fight on. Until Pearl Harbor, there was strong isolationist pressure to stay out of it, or even to side with Germany. After all, Britain, not Germany, was our big commercial rival. And Germany's enemy, the Soviet Union, was everybody's ideological bogeyman.

In those days, war wasn't considered good for business. It meant government control of industry, a shortage of labor (rising wages) and of raw materials, transport, and so on (rising costs). It meant
loss of markets, price and wage controls, interrupted trade, and neglecting lucrative consumer trade for government managed contract work.

Roosevelt's instinctive revulsion to Fascism and support for Britain was not very popular in the business community.