Nazis were invading other places now. The US cut off Japan's oil supply, triggering an existential crisis for Japan that led to a military attack.

If United States had joined the Axis powers, leadership very plausibly could have continued the practice of isolationism (which was the popular opinion before Pearl Harbor). It might have been a shortsighted move, but Nazis invading Europe did not sell the US population on entering the war.

Comment I replied to was talking about Europe during WWII