Between the 5th and 4th centuries BCE, Celtic tribes moved en masse into southern Europe, intent on seizing land and wealth to feed their swelling numbers. As these tribes began crossing the Alps, they came into conflict with the Romans and Greeks living around the Mediterranean. The death of Alexander the Great (356 BCE - 323 BCE) made Greece appear like an easy target to many opportunistic Celtic chieftains who led their people into the Balkans, first to Thrace and Illyria before pressing forward towards Macedon.
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/1401/the-celtic-invasion-of-greece/.