I would say World War 2 and the reason why is that the world was threatened by one countries dominance. Germany wanted to rule the world and they came mighty close to achieving it. Europe fell too quickly and weren't prepared. If it wasn't for the Russians , English and the Americans the world would be a total different place today.

There are many important battles through the ages but this one stands out for me as it was crucial for the world powers to come together. This will make any country think now that they arent fighting one country but others will get involved. keeping world peace is crucial to the stability of the world.