Antiochus sent an expeditionary force into northern Greece hoping to win local Greek support and overthrow the Roman rule there. The reaction from the Greeks was disappointing at best. The recently defeated Philip of Macedon threw in his lot with the Romans, and only the Aetolians seem to have been forthcoming in any numbers. The arrival of a force of 20,000 Romans to intercept the Seleucids threw Antiochus into retreat. Seeking a suitable location to hold off the superior Roman numbers, his forces fell back on the famous pass of Thermopylae. This had of course played a much more famous role in attempting to halt the Persian invasions several centuries earlier.
The pass at Thermopylae had sea on one side, with marshy ground and even quicksand nearer the water, and steep and seemingly impenetrable mountains on the other side. Antiochus fortified the narrow pass by adding palisades and walls,
