Thermopylae (Θερμοπύλαι; "Hot Gates"): small pass in Greece, site of several battles.
After the death of Alexander, his generals created states of their own. One of the most successful leaders was Lysimachus, who, after the Battle of Ipsus, was in control of Thrace, large parts of Asia Minor, and substantial parts of Greece. However, he was defeated by Seleucus I Nicator in 281 (Battle of Corupedium), and the victor was in turn assassinated by Ptolemy Keraunos.
This meant that Lysimachus' kingdom disintegrated, and that Greece was no longer protected against the tribes in the north, the Galatians. These Galatians belonged to the La Tène culture, which is often called "Celtic"; and they found the way to the south open.
In 279, the Greeks made a last stand at Thermopylae, but they were defeated after the Galatians had taken the same path as the Immortals, two century before. The full story is told by Pausanias.note[Pausanias, Guide for Greece 10.19.4-23.9.]