Charles Ellicott Commentary 2 Samuel 5:3

"So all the elders of Israel came to the king to Hebron; and king David made a covenant with them in Hebron before Jehovah: and they anointed David king over Israel." — 2 Samuel 5:3 (ASV)

Made a league with them. —It would be an anachronism to interpret this as the establishment of a constitutional monarchy, but the “league” may have pertained to certain special matters, such as leading them against their enemies, not destroying the remnant of the house of Saul or its late adherents, and not showing partiality (as Saul had done) to the members of his own tribe.