Mark Antony was the Roman triumvir who married Octavia after beginning his relationship with Cleopatra. The sequence is central to the political drama of the late Roman Republic.
Antony met Cleopatra in 41 BC at Tarsus, and their relationship soon produced twins, Alexander Helios and Cleopatra Selene. In Rome, Antony's alliance with Octavian was deteriorating, so Antony married Octavian's sister Octavia in October 40 BC. The marriage was both personal and intensely political: it was intended to stabilize the uneasy partnership between the two triumvirs.
Octavia was regarded as a loyal wife and later acted as a mediator between her husband and brother. Antony, however, returned to Cleopatra in the East. He eventually divorced Octavia in 33 BC, and the rivalry between Antony and Octavian culminated in the war that ended with Antony and Cleopatra's defeat at Actium and their deaths in 30 BC.
The common confusion is treating Octavia as Cleopatra's rival in a simple romantic triangle. Their marriage also served Roman statecraft and factional politics.