Final answer:
The power struggle between Julius Caesar and Pompey was marked by their transition from allies in the First Triumvirate to adversaries in a civil war, resulting in Caesar's rise to power after defeating Pompey in the Battle of Pharsalus and Pompey's subsequent murder in Egypt.
Step-by-step explanation:
The power struggle between Julius Caesar and Pompey the Great was a defining moment in the late Roman Republic's history. Initially, both men belonged to the First Triumvirate, an informal political alliance with Crassus designed to dominate Roman politics. After Crassus's death, Pompey aligned with the Senate's optimates, creating a significant rift between him and Caesar. In 49 BCE, Caesar's refusal to disband his army and his subsequent march on Rome initiated a civil war. The conflict culminated in the Battle of Pharsalus in 48 BCE, where Caesar defeated Pompey's forces. Pompey escaped to Egypt only to be murdered upon arrival, as the Egyptians sought favor with Caesar, who emerged as the supreme power in Rome and began an affair with Cleopatra VII.