Alexander I Theopator Euergetes, surnamed Balas, was the ruler of the Seleucid Empire from 150 BC to August 145 BC. Picked from obscurity and supported by the neighboring Roman-allied Kingdom of Pergamon, Alexander landed in Phoenicia in 152 BC and started a civil war against Seleucid King Demetrius I Soter. Backed by mercenaries and factions of the Seleucid Empire unhappy with the existing government, he defeated Demetrius and took the crown in 150 BC. He married the princess Cleopatra Thea to seal an alliance with the neighboring Ptolemaic kingdom. His reign saw the steady retreat of the Seleucid Empire's eastern border, with important eastern satrapies such as Media being lost to the nascent Parthian Empire. In 147 BC, Demetrius II Nicator, the young son of Demetrius I, began a campaign to overthrow Balas, and civil war resumed. Alexander's ally, Ptolemaic king Ptolemy VI Philometor, moved troops into Coele-Syria to support Alexander, but then switched sides and threw his support behind Demetrius II. At the Battle of the Oenoparus River in Syria, he was defeated by Ptolemy VI and he died shortly afterward.

The Seleucid Empire was a Greek power in West Asia during the Hellenistic period. It was founded in 312 BC by the Macedonian general Seleucus I Nicator, following the division of the Macedonian Empire founded by Alexander the Great, and ruled by the Seleucid dynasty until its annexation by the Roman Republic under Pompey in 63 BC.