Hello there @Goldsree999
The main reason for the aggression across Europe was because the Nazis (at the time) and the Japanese wanted to expand and take over large countries like Europe. That was the main reason that Japan bombed Pearl Harbor (to immobilize America's Pacific fleet of ships, so America couldn't stop the Nazis from taking over Europe)
Let me know if there is anything else I can help with!
I hope I helped
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