2 Samuel 5:3 - Ellicott's Commentary On The Whole Bible

Made a league with them. — It would be an anachronism to understand this of the establishment of a constitutional monarchy, but the “league” may have had reference 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.