Personally, I think the 1912 election was the most important in American History. Because of my interest in it I decided to make a video discussing it. Feel free to point out anything I got wrong! : HistoryWhatIf
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What If Teddy Roosevelt Won A Third Term - YouTube

In our timeline, Teddy Roosevelt lost the Republican nomination that year, and he ran as the candidate of the Progressive or Bull Moose Party. He ended up splitting the Republican vote, and Democrat Woodrow Wilson won.

But if he had gotten nominated, and if he had won, then what then?

That video shows an editorial cartoon of Europe at the beginning of the Great War, as WWI was originally called.
  • A Briton says "Ho there, if you chaps..."
  • A Frenchman says "If you hit him..."
  • A German says "If you strike my friend I'll..."
  • A Russian says "If you hit that little feller I'll..."
  • An Austrian says "If you make a move I'll..."
  • A Serb says "If you touch me I'll..."

All in stereotypical outfits, like the German with a spike helmet.

In our timeline, Pres. Wilson tried to avoid getting involved, as TR commented from the sidelines quite a lot. But in this timeline, TR gets ready for war, and when the Germans sink the Lusitania, he starts US involvement. He orders as many as 3 million US soldiers into Europe. This tips the balance against the Central Powers: Germany, Austria-Hungary, Bulgaria, and the Ottoman Empire.

The Ottoman Empire and Austria-Hungary would suffer rebellions and break apart, leaving Germany as the strongest Central Powers state. But Russia has some success against Germany, and that staves off the Bolshevik Revolution. When Germany surrenders, TR has a much more conciliatory policy toward the defeated nation than the Western Allies do in our timeline. This keeps Nazism from getting widespread support.

The author expects the Bolsheviks to try to take over Russia -- and for TR to send in some troops to squash them.

Domestically, the US gets ahead of our timeline on a lot of progressive policies, like women having the vote and New Deal stuff like Social Security.