alternatehistory.com

It could be argued that America’s decision to enter WWI led to...

-Russia staying in the war longer, resulting in Communism taking over Russia and eventually much of the world where it resulted in the death of the better part of a hundred million people.

-Germany turning to Nazism and starting a war and genocide that killed 40+ million

-Collapse of Austria-Hungary and destabilization of Central Europe and parts of the Balkans which could have fallen under Austrian/Bulgarian domination (maybe not any better, but multiple genocides happened OTL that could maybe be avoided)

-The Ottoman Empire collapsing, which has arguably destabilized the region
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