Charles Ellicott Commentary Genesis 14:4

"Twelve years they served Chedorlaomer, and in the thirteenth year they rebelled." — Genesis 14:4 (ASV)

They served. —That is, paid a yearly tribute, that they might be exempt from Chedorlaomer’s marauding expeditions (see 2 Kings 18:7). There must, therefore, have been envoys going from time to time to and from the Jordan valley to Shinar.