'Rashi' (15:19, citing Seder Olam): Shlomo married Bas Pharaoh in the fourth year of his reign. ¹ He reigned 40 years in all. It was decreed that Aram will harass Yisrael for 36 years, corresponding to the 36 years that he was married to her, and in the end they will fall to Bnei David. This is why it says the 36th year of Asa (it was 36 year after Shlomo died). In the 16th year of Asa, after Zerach ha'Kushi fell, Melech Yisrael and Melech Aram made a Bris to fight Asa. Asa erred, and sent gold and silver to Melech Aram, and asked to make a Bris with him.

Radak: Sefer Melachim does not count from the start of Asa's reign, rather, from when he had wars, for it discusses also Malchei Yisrael. Asa had 10 years of serenity at the beginning of his reign (13:23). Two of those 10 years were the last years of Yaravam. Nadav did not rule until eight years later. Or, he ruled, but his kingship was weak, and the years are not counted towards his reign. In the second year of Asa, i.e. after the 10 years of serenity, Nadav became king and fought Asa. In Asa's third year ² Basha became king, and they fought all their days (Melachim I, 15:16). This war in which Basha built Ramah was in Basha's 23rd year, which was the 36th year from the beginning of Asa's reign, and the 25th or 26th year of his wars. One year of Nadav was counted also for Basha, for perhaps Nadav did not complete the year.

Radak citing Seder Olam: This war in which Basha built Ramah was in Asa's 16th year, one year after his war with the Kushim (Divrei ha'Yamim II, 14:8-14). Malchus Beis David was destined to return then; it was the 36th year from Shlomo's death. Asa messed up, and relied on Melech Aram, and sent to him Otzros Beis Hashem. Since he did not rely on Hashem, Malchus Beis David did not return. It remained divided. ³

Refer to 15:19:1:3.

Malbim: He saw that many from his kingdom were going to Asa's kingdom (15:9); he wanted to stop this. He built Ramah to close his land and block traffic to and from Yehudah.