Prevent the formation of Islam.
Okay, and I'll prevent the formation of Christianity. Between us we will have done more to aid mankind than any other people in history.
Prevent the formation of Islam.
You'd be doing the world of math and science a great disservice:
http://en.wikipedia.org/wiki/Science_in_the_medieval_Islamic_world
Would probably go back and tell my parents not to bother
I would prevent the war of northern aggression. I would take with me all the hitory/strategy guide I had on the subject and try to warn Lee about the enemies strategy. I would also bring as many weapons(and ammo) as I could fit into my time machine.
My aim, then, was to whip the rebels, to humble their pride, to follow them to their inmost recesses, and make them fear and dread us. Fear is the beginning of wisdom.
I would make this war as severe as possible, and show no symptoms of tiring till the South begs for mercy.
Considering what Islam does to stifle math and science now it's probably a wash or even a net gain.
And if we eliminate what Christianity did to hold back advancements in math and science the world would be a couple of hundred years ahead.
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