now weakened, the Seleucid Empire became increasingly aggressive and successful in its attempts to conquer the entire Greek world. When Rome pulled out of Greece at the end of the Second Macedonian War, they (and their allies) thought they had left behind a stable peace. However, by weakening the last remaining check on Seleucid expansion, they left behind the opposite. Now not only did Rome's allies against Philip seek a Roman alliance against the Seleucids, but Philip himself even sought an alliance with Rome. The situation was made worse by the fact that Hannibal was now a chief military advisor to the Seleucid emperor, and the two were believed to be planning for an outright conquest not just of Greece, but of Rome also. The Seleucids were much stronger than the Macedonians had ever been, given that they controlled much of the former Persian Empire, and by this point had almost entirely reassembled Alexander the Great's former empire. Fearing the worst, the Romans began a major mobilization, all but pulling out of recently pacified Spain and Gaul. They even established a major garrison in Sicily in case the Seleucids ever invaded Italy. This fear was shared by Rome's Greek allies, who had largely ignored Rome in the years after the Second Macedonian War, but now followed Rome again for the first time since that war. A major Roman-Greek force was mobilized under the command of the great hero of the Second Punic War, Scipio Africanus, and set out for Greece, beginning the Roman-Syrian War. After initial fighting that revealed serious Seleucid weaknesses, the Seleucids tried to turn the Roman strength against them at the Battle of Thermopylae (as they believed the 300 Spartans had done centuries earlier to the mighty Persian Empire). Like the Spartans, the Seleucids lost the battle, and were forced to evacuate Greece. The Romans pursued the Seleucids by crossing the Hellespont, which marked the first time a Roman army had ever entered Asia. The decisive engagement was fought at the Battle of Magnesia, resulting in a complete Roman victory. The Seleucids sued for peace, and Rome forced them to give up their recent Greek conquests. Though they still controlled a great deal of territory, this defeat marked the beginning of the end of the Seleucid empire, as they were to begin facing increasingly aggressive subjects in the east (the Parthians) and the west (the Greeks), as