Trump: I am strongly considering pulling out of NATO - Telegraph

US didn't really define its mission. They kept it very fluid. Besides in which war did the sides define their mission and managed to accomplish it exactly like they planned?

I wonder what would have happened if US kept the war going only with Japan?

This would have been a possibility:

If the U.S. had fought only Japan, the Pacific war likely would have ended sooner, but Germany would have likely defeated or forced a stalemate with the Soviet Union and Britain in Europe. Germany would have concentrated its massive military forces on the Eastern Front without U.S. Lend-Lease aid, potentially allowing them to seize control of the continent.

Like I said, you Europeans are really arrogant and think the world owes you something.

Well that arrogance cost 20 million lives ; maybe they learnt a lesson in amongst the conflict of war .

I don't think it's arrogance at all.

It was the survivors of WW1 and WW2, noting the arrogance of princes and generals, who put the core institutions in place to prevent another global war (NATO, UN, ICC, EU etc.) . People who lived through mechanised slaughter, whole generations were wiped out, but very few princes and generals (elites) were affected.

Towns, villages and families across Europe still carry the scars - memorials to a lost generation, death camps, mass graves, and family trees missing a generation of young men.

The truth...war is a very profitable business.