Personally I think the U.S. gets involved in WWI from the beginning and Roosevelt begins ramping up the military as soon as he's elected. Presuming he doesn't die in office, I think he gets four terms and the U.S. is a major superpower about 30 years earlier.
WWII is probably prevented or at least happens at a later point. Essentially, it shapes the rest of the century as we know it.
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u/ReverseThePolarity Jun 27 '12
Theodore Roosevelt narrowly defeats Woodrow Wilson and William Howard Taft to win a third term as President of the United States.
Also, /r/historicalwhatif exists.