Amateur time travelers always go back in time and kill Hitler.  But I think the entry of the US into WW1 was even worse than Hitler’s rise of power — because they actually led to the circumstances that made Hitler possible.  Without the punitive Versailles treaty made possible by the US breaking the trench warfare stalemate, Hitler is just a simple WW1 veteran.  You can kill two birds with one time-travelling stone!  The other key point (from the same year, in fact, the US declaration of war and Lenin’s trip to Russia occurred the same week!) that results in almost untold suffering was the export of Lenin to Russia, where he transformed a potentially moderate revolution into an extremist one.  So, if you only have one time machine, you might want to consider going back and stopping Lenin too.

But I’ve just come across the holy grail of time travelling history revisionism: Arthur Zimmerman.  Yup, astute high school history students will remember that he was the author of the Zimmerman telegram, the German message to Mexico asking them to (!!!) declare war on the US and reclaim some southwestern states.  Thankfully, the Mexican leader smartly declined this opportunity, but not before the Americans became convinced that this constituted casus belli.  It’s not an exaggeration to state that without this revelation, the US may not have entered WW1.  But here’s the kicker: Zimmerman, in his role as foreign secretary, was also personally responsible for that other terrible historical event:

In March 1917, with the imminent collapse of the Russian front, Zimmermann took steps to promote Peace in the East with the Russians, a proposal that was of immense importance to Germany at the time. The foreign secretary set forth the following: regulations for frontline contacts with the opposite side; reciprocal withdrawal of the occupied areas; an amicable agreement about Poland, Lithuania and Kurland; and a promise to aid Russia in its reconstruction and rehabilitation.

Last not least, Lenin and the émigré revolutionaries would be allowed to pass through Germany to Russia by train. These proposals once carried out, would free Germany’s armies in the east and allow them to be concentrated in the west, a master-stroke that would reinforce the German western front vastly. Zimmermann thus contributed to the outcome of the October Revolution.

So, aspiring time travelers, if you’ve only got one paradox-causing bullet to shoot, here’s your target.