The assassination of Archduke Franz Ferdinand which triggered world war 1 even though there was tension before it.The casualties in this war was 40 million a population in some countries around the world which is mind blowing even when i try imagining it.
Yeah, this is the trigger, though it is said that the German Empire has already plans for war against the Russian Empire sometime around in 1914-1915, before the Russians modernize and present a threat to the then economically-powerful Germans.
Though, if you'll ask me, another watershed moment in history is the Defenestration of Prague in 1618, which eventually triggered the Thirty Years' War, and resulted in the Treaty of Westphalia. That war did trigger the people to ask, and eventually criticize, the 'divine mandate' of monarchs in Europe.
Another case would be the Sack of Baghdad during the Mongol Conquest of the Abbasid Caliphate. The fateful event was so traumatic, it got embedded in the memories of Muslims then and now (the end of the Islamic Golden Age). It is said that this event became one of the reasons for the 'conservative radicalism' of some of the extremists that lasted today.