The story of Baasha actually begins with his immediate predecessor King Nadab, son of King Jeroboam (1 Kings 15:25). Baasha comes to power through a coup d’état, an assassination attempt on Nadab. Apparently Baasha has allies in the army with whom he conspires. Gibbethon is a city in Philistine territory, which was assigned to the tribe of Dan at the conquest of the land of Canaan. But the Danites lost this territory (Joshua 19:44, Joshua 19:47). In Joshua 21:23 it says that Gibbethon became a Levite city. In the time of King Baasha, Gibbethon was a Philistine city.
27 Baasha the son of Ahijah, of the house of Issachar, conspired against him. And Baasha struck him down at Gibbethon, which belonged to the Philistines, for Nadab and all Israel were laying siege to Gibbethon.