Julius Caesar (44 BC)

The assassination of Julius Caesar by 23 senators on the Ides of March, 44 BC, ended the Roman Republic and inaugurated an imperial system that governed a quarter of humanity for 500 years. The ensuing 13 years of civil war destroyed the republican institutions that had held Rome together for nearly five centuries. More than any other assassination on this list, Caesar's death marks a clear pivot from one era to another: the Republic never recovered, and his adopted heir Octavian became Augustus, the first emperor. Within 50 years, Rome shifted from a representative system to autocracy, demonstrating how a single murder can alter the political trajectory of a civilization.