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 got to 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). 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. Although they still controlled a great deal of territory, this defeat marked the decline of their empire, as they were to begin facing increasingly aggressive subjects in the east (the Parthians) and the west (the Greeks). Their empire disintegrated into a rump over the course of the next century, when it was eclipsed by Pontus. Following Magnesia, Rome again withdrew from Greece, assuming (or hoping) that the lack of a major Greek power would ensure a stable peace. In fact, it did the opposite.
What loss started a decline of the Seleucids Empire?
Battle of Magnesia