Why did the northern kingdom decline before Judah?
The northern kingdom declined first because Jeroboam immediately established idolatrous worship and led the ten tribes into abject idolatry after the nation divided. That spiritual downfall accelerated over hundreds of years, culminating in the fall of the northern tribes around 740–750 B.C., while Judah followed about 150 years later.
Les teaches that once the nation divided, the first part of Israel to go into abject idolatry was the northern kingdom under Jeroboam. He immediately set up idolatrous worship and immediately led the northern ten tribes into idolatry, so they would go down the banana peel first. About 150 years later, Judah would follow. By the time of 2 Kings chapter 17, much time had passed and the years were piling up; this was the time of the fall of the northern tribes, about 740 or 750 B.C. Over hundreds of years they had been going down and down. The nation had been divided with a separate government and religious system in Samaria, and Syria to the northeast had always been a mortal enemy of Israel.