Except that Union victory ended around 850k deaths, emboldened US interventionism, ultimately in WW1, leading to the rise of Hitler, the Holocaust, etc. If the Union had only let Ft Sumter go and the Confederacy fail or reform on its own, what lesser human toll overall would it have been?

Europe didn't need the US's help for their own militarisation and expansion of empires, or for Archduke Franz Ferdinand to get shot. And as long as we're playing "what if", you can easily imagine a divided US where slavery was even more ingrained, ignoring Nazi Germany for even longer

The only mistake the US made during the civil war was not summarily executing every confederate officer, government official, and plantation owner