Jamieson, Fausset and Brown on 1 Kings 15:16-17

16-17. there was war between Asa and Baasha king of Israel all their days--Asa enjoyed a ten years' peace after Jeroboam's defeat by Abijam, and this interval was wisely and energetically spent in making internal reforms, as well as increasing the means of national defense (2Ch 14:1-7). In the fifteenth year of his reign, however, the king of Israel commenced hostilities against him, and, invading his kingdom, erected a strong fortress at Ramah, which was near Gibeah, and only six Roman miles from Jerusalem. Afraid lest his subjects might quit his kingdom and return to the worship of their fathers, he wished to cut off all intercourse between the two nations. Ramah stood on an eminence overhanging a narrow ravine which separated Israel from Judah, and therefore he took up a hostile position in that place.