I say WWII - it brought groundbraking changes in society, politics, science, culture and human interrelationships and interaction. I do not mean the war itself, but rather everything wich came to be during and right after it. Woman equality, racial equality, a stop to eugenics, decolonization, global stability if not peace, we didn't blew ourselves out during the cold war, giant leaps in technology and transportation, the United Nations etc. If it weren't for that BIG war, we would had 1000s of SMALL ones, European states would still fight each other, Asia would probably be embroiled in a large conflict if not civil war, fuelled by Yanks and Comms, Africa would be a playground and soceity wouldnt be so liberal as today.