During the prelude to the Roman-Seleucid War, Seleucus was put in charge of the re-established colony of Lysimacheia by his father. Upon the outbreak of war, Seleucus commanded his own force, unsuccessfully besieging Pergamon, and taking the city of Phocaea before fighting in the Battle of Magnesia alongside his father. After their defeat at Magnesia, Seleucus was made co-regent in 189 BC and the Seleucids signed the Treaty of Apamea with Rome in 188 BC. As part of the treaty, Seleucus oversaw the supply of grain and scouts to Roman and Pergamene forces during their campaign against the Galatians.