In the third century BCE, the ancient kingdom of Macedon held dominion over mainland Greece, but it was rapidly descending into chaos. After Alexander the Great's death, several of his successors contended for the Macedonian throne, and amid the tumult the Celts launched a massive invasion, ravaging and plundering Macedon and northern Greece for years. The Celts finally met their defeat at the hands of Antigonus Gonatas, son of one of Alexander's...