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
-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