The key event that marked the transition from the Roman Republic to the Roman Empire was Julius Caesar's crossing of the Rubicon River in 49 BCE. This act was a bold defiance of Senate authority and initiated a civil war between Caesar and Pompey, ultimately leading to Caesar's dictatorship. Following his assassination in 44 BCE, a power struggle ensued, culminating in the rise of Caesar Augustus (Octavian) after defeating Mark Antony and Cleopatra. This transition transformed Rome from a republic with elected officials to an autocratic empire ruled by emperors, fundamentally altering the structure of Roman governance and society.