This is a really difficult question, because Europe is the craddle of civilization, almost everything that happened there was important. But for modern history i would say two (closely realted): the French Revolution (and all it could have been if the buorgeoisie didn't take the power) and the creation of the Commune of Paris, one of the most relevant evidence that communism works (sure the central power on Paris wiped them out of the planet when they changed their minds and wanted them under their dominion...)