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. Nadab and his entire army were besieging Gibbethon, and there Baasha killed him. The writer does not tell us, but possibly Baasha was a commander of some portion of the army in which he had some support. We also might suggest that Nadab’s hold upon the army was somewhat loose, for it is hard to understand that a loyal army surrounding a king would allow him to be assassinated.
Baasha is the son of Ahijah, which the writer further defines as of the house of Issachar.
We should not confuse this Ahijah with the prophet by the same name. That Ahijah was called the Shilonite, referring to Shiloh as his place of residence.
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.