“Now sons were born to David.” The Law of Moses warns kings not to take many wives (Deut. 17:17), and the fact that David started his kingdom with six is somewhat troubling. Although it was important for the wife of a king to have sons who could take over the kingdom, sons born by different wives to a king almost always meant trouble because each son would not only be supported and promoted by the mother, but by the whole clan, tribe, or kingdom from which the mother and son came. Thus it was common in the ancient world for the sons of kings to murder each other or otherwise be in conflict. David’s household was no different: Amnon raped his half-sister Tamar; Absalom murdered Amnon then later rebelled against David and was killed; and Adonijah was executed by Solomon for conspiring to take the kingdom. Not a happy family.
“his firstborn was Amnon, of Ahinoam the Jezreelitess.” Very quickly after being anointed king over the tribe of Judah (2 Sam. 2:4), David had six sons by six different women (2 Sam. 3:2-5). It is unlikely that all David’s wives had only sons, suggesting that this list is more to show the strength of the kingdom than to give a full representation of David’s family. Scripture had just said David’s house was getting stronger (2 Sam. 3:1), and a king having sons was one way that happened. The diversity of David’s harem supports the conclusion that he was marrying for political reasons. David had not yet gained control of all Israel and needed a broad base of followers and allies to succeed. “Ahinoam the Jezreelitess” was from the “Jezreel” of Judah, not the Jezreel in the Jezreel Valley. This Jezreel is in south-central Judah, not far from Maon, Ziph, and Carmel (see commentary on Josh. 15:56). So Ahinoam and Abigail were both from the hill country of Judah, which was Davids's birth territory, and those marriages solidified his friend and family base in that area.
Another of David’s wives was Maacah the daughter of Talmai king of Geshur (2 Sam. 3:2). She was not an Israelite. Geshur was a territory just north of the territory conquered by the tribe of Manasseh in the Transjordan (east of the Jordan River). Geshur was not conquered by Israel during the time of Joshua and remained independent. Since Geshur was just north of the territory controlled by Saul’s son Ish-bosheth when he set up his capital in the Transjordan in Mahanaim (2 Sam. 2:8), David’s marriage to Maacah assured him that Ish-bosheth would not secure military allies from the region north of him. Years later however, when Absalom, David’s son by Maacah, murdered his brother Amnon, Absalom fled to his grandfather Talmai king of Geshur who protected him (2 Sam. 13:37).
The fact that David married a non-Israelite for political and military reasons may have seemed wise at the time, and certainly seemed to pay off in his war with Ish-bosheth, but it certainly also may have set a bad precedent for his family. Solomon married a non-Israelite before he even became king. He married Naamah the Ammonite who gave birth to Rehoboam, and Rehoboam became the king of Judah after Solomon died (1 Kings 14:21). Solomon went on to marry many non-Israelite women, and they greatly contributed to his downfall in life (1 Kings 11:4).