Antiochus III was barely 19 when he became king of Seleucia. By a show of force, and skilful diplomacy, he formed alliances with neighbouring rulers. Once he had his neighbours under his control, Antiochus then turned his attention to Western territories and formed an alliance with Philip V of Macedon against Egypt. In 200 BC, he defeated the Egyptian forces, and claimed control of Palestine and Phoenicia.
Pressing his military advantage, Antiochus invaded Egypt itself, forcing a peace treaty. But Antiochus did not rest on that victory. He later led his forces to regain the cities of Western Asia Minor that his father had lost. Not content with having extended Seleucia’s control over more territory than any of his predecessors, he crossed into Europe, but was defeated by the Romans and the allies.