The Boston tea party, of december 16th, 1773.

This was a decisive military action against the English, which basically was the start of the United States as we know it.

As the US is now leading the Western world, I believe we can clearly state that this battle has had a great impact on the world as we know it, and on a lot of the events that have taken place since on world scale. 

For instance:

How would WWII have finished, if the Boston tea party had never taken place? 

Even the battle of Waterloo (I am currently about 15km away from Waterloo (Brussels, Belgium) while writing this btw ;-) ) would've been different, or not have taken place, if the Boston tea party would not have happened (the army of the British Duke of Wellington was at about half force when going to battle against Napoleon, because a large part of his army was still in the US, or had died there).