Albert Barnes Commentary 1 Kings 15:17

"And Baasha king of Israel went up against Judah, and built Ramah, that he might not suffer any one to go out or come in to Asa king of Judah." — 1 Kings 15:17 (ASV)

Ramah (perhaps “Er-Ram,” as noted in the margin) was located halfway between Bethel and Jerusalem. Its distance from Jerusalem was no more than five miles, so its occupation was a menace to the capital.

Baasha’s seizure of Ramah implies he had previously recovered the towns taken by Abijam from Jeroboam—namely, Bethel, Jeshanah, and Ephrain (2 Chronicles 13:19)—and was now carrying the war into the enemy’s country. If his conquest had been maintained, it would have seriously crippled Judah and almost compelled the capital to be moved to Hebron.

That he might not suffer any to go out or come in—Baasha, in seizing Ramah, professed to be acting defensively. His complaint seems to have been well-founded (compare 2 Chronicles 15:9), but this was more than a defensive measure; it was the first step toward conquering the southern kingdom.