After being appointed governor of Rome’s northern territory of Gaul in 58 B.C., Caesar vastly extended the boundaries of the Roman Republic across Europe, all the way to the shores of the Atlantic Ocean and English Channel. During the bloody eight-year Gallic Wars, his legions conquered local tribes in present-day France, Belgium and Switzerland.

In 55 B.C., Caesar’s army built a timber bridge spanning the Rhine River in just 10 days—a marvel of military engineering. Then, Roman troops marched across the waterway for the first time ever to subdue German tribes threatening eastern Gaul. Later that year, the Roman general commanded the first of two expeditions across the English Channel. These initial incursions into Britain laid the foundation for Rome’s eventual conquest of much of the island.

Through his masterful battlefield tactics and willingness to fight in combat, Caesar earned the respect and loyalty of his soldiers. He also earned the jealousy of Pompey, his former political ally in the First Triumvirate who effectively ran Rome. When Pompey’s supporters in the Roman Senate demanded that Caesar disband his army and return to Rome as a civilian, he refused.

Instead, in 49 B.C., Caesar brazenly led a legion across the Rubicon River, which divided Gaul and Rome, sparking a civil war. Forces supporting Caesar on one side and Pompey on the other battled as far away as Spain, Greece and North Africa. Although outnumbered, Caesar’s legions defeated Pompey’s army in a decisive battle at Pharsalus, Greece, in 48 B.C., prompting Pompey to flee to Egypt. But before he could even step ashore, he was assassinated at the behest of the teenaged pharaoh Ptolemy XIII. Ptolemy, it turned out, sought Caesar’s support in Egypt’s own civil war—one that had pitted him against his co-regent and sister, Cleopatra VII.

When Roman reinforcements arrived in early 47 B.C., Caesar’s forces defeated Ptolemy’s army in the Battle of the Nile. After the young king drowned in the Nile River while fleeing the battle, Caesar installed Cleopatra and her 12-year-old half-brother, Ptolemy XIV, as co-regents, but the queen held the true power.

But Ptolemy gravely miscalculated. When he presented Caesar with Pompey’s severed head upon his arrival in Alexandria, the reaction wasn’t gratitude; it was horror. Caesar promptly executed Pompey’s assassins and sided with Cleopatra in Egypt’s civil war.

According to the ancient historian Plutarch, Caesar had the 21-year-old Cleopatra transported into the royal palace where he had taken residence by having her smuggled inside a linen sack filled with dirty laundry. As urban warfare erupted in Alexandria, the pair began a romance while besieged in the palace for six months.

Around the time Caesar returned to Rome, Cleopatra gave birth to a boy believed to be his son. The Egyptian queen named him Ptolemy XV, but Alexandrians mockingly referred to the boy as Caesarion, “Little Caesar.”In 46 B.C., Cleopatra and her infant son moved into a villa on the banks of Rome’s Tiber River, and the married Caesar continued to visit his mistress in apparent violation of Rome’s bigamy laws.

Following Caesar’s murder, Cleopatra returned to Egypt and later had a love affair and alliance with Caesar’s deputy, Mark Antony. The pair committed suicide after Augustus defeated their armies in 31 B.C.

Buoyed by the support of his army and Rome’s plebeians (non-elite citizens), Caesar emerged from his war with Pompey with tremendous power. After being named dictator of Rome for 10 years in 46 B.C., he declared himself “dictator for life” the following year. Caesar’s sweeping reforms—such as granting property to retiring soldiers, redistributing land to the poor and canceling debts—proved popular with the military and Rome’s lower and middle classes.

Caesar’s reforms angered elites, as did his disregard for the Roman Senate and republican tradition. A cult of personality developed around Caesar as he minted coins with his image, celebrated his birthday as a public holiday and ruled the Senate from a golden throne.

Caesar’s autocratic rule heralded the dawn of the Roman Empire. Dozens of senators who believed Caesar’s concentration of absolute power threatened the republic’s democratic institutions plotted his murder, which occurred on the Ides of March in 44 B.C.

Rather than saving the 400-year-old Roman Republic, however, the assassination accelerated its demise. In their attempt to thwart a dictator, the senators inadvertently created an emperor. Caesar’s heir, Augustus, emerged from a lengthy civil war as Rome’s supreme leader after purging his enemies, murdering Caesar’s assassins and cracking down on republicans. Augustus deified Caesar (effectively making Augustus the son of a god) and ushered in the autocratic Roman Empire, which lasted for approximately five centuries.

Caesar was so powerful that he changed time. The traditional Roman calendar, which was based on the 355-day lunar year, required constant revisions since it fell out of sync with seasons and festivals. Aided by the Greek mathematician and astronomer Sosigenes, Caesar enacted the Julian calendar, which was based on the 365¼-day solar year with a “leap day” added every four years.

To reboot the calendar, 46 B.C. spanned 445 days before Caesar’s system took effect on January 1 in 45 B.C. The Julian calendar predominated in most of the Western world for 16 centuries. Since each solar year is slightly less than 365¼ days, the Julian calendar gained one day every 131 years. By the 1500s, the calendar was 10 days out of step with the seasons. In 1582, Pope Gregory XIII modified the calendar by eliminating 10 days from that year and decreeing that only one out of every four centennial years would be leap years. The Julian calendar is still in use in portions of the Eastern Orthodox Church.

An additional legacy of Caesar can be seen on the calendar. Following Caesar’s assassination, the name of his birth month was changed from Quintilis to Julius (July) in his honor.