1Ki 2:4 that the LORD may fulfill His word which He spoke concerning me, saying, ‘If your sons take heed to their way, to walk before Me in truth with all their heart and with all their soul,’ He said, ‘you shall not lack a man on the throne of Israel.’
Note the order:
1: walking in his ways (vs 3, NRSV)
2: The LORD keeps his promise (vs 4)
The original promise seemed to be unconditional (2 Samuel 7:11-16); so how can an unconditional promise be conditional?
In my view, all divine promises are conditional. It is born out often through Israel’s history. See Deuteronomy 4:29; 9:5, etc.
Nevertheless, it seems the conditional promises in the book of Kings apply to Solomon’s actions, which he violated (1 Kings 11). So the conditional part may apply to a dynasty of kings over the entire nation of Israel (or, particularly, the zone of the northern tribes) – that was lost due to Solomon’s failures. In other words, the promise to David in 2 Samuel 7:11ff is unrelated to the covenant to the dynasty regarding the northern kingdom.
Note that Jeroboam was given a similar conditional promise of a dynasty (1 Kings 11:38).
God partly maintained his promise to David (1 Kings 15:4; 2 Kings 8:19) but since the covenant had been broken, God was not compelled to uphold any of his promise to David.
consider Isaiah 24:5, 18 in which the “rainbow” covenant of Genesis 9:16 was (it turns out) conditional (or, two-sided).