Actually the wrong fork in the road was in 1588 at the defeat of the Spanish Armada. That led to British ascendency in the New World, instead of Spanish.
If that battle had gone the other way all us Yanks would be speaking Spanish instead of English, and there would be no worries over illegal immigration.

Of course, if you really want to be pedantic, you could claim the greatest influence on our history was the loss of the Battle of the Teutoberg Forest - when Hermonius clobbered Quintillius Varus during the reign of Caeser Augustus. That stopped Roman expansion at the Rhine in what is now Western Europe.

Anybody else got any other ideas?
Mark