Why does the Torah mention the fact that the five kings served Kedorla'omer for twelve years?
Seforno: This is in fact, a continuation of the previous Pasuk, which informs us that Amrafel and the two warring factions met in the valley of Sidim and arrived at a compromise - namely, that the five kings should serve the three. 1 It was in the thirteenth year of this arrangement that they rebelled.
QUESTIONS ON RASHI
Rashi writes: "'In the fourteenth year' - of their rebellion." Perhaps the rebellion lasted only one year (the thirteenth), and the war took place in the fourteenth year since their servitude?
Gur Aryeh: The verse states, "u'Shelosh Esreh Shanah Maradu - they rebelled for thirteen years." It does not say "u'vi'Shelosh Esreh Shanah, - in the thirteenth year." If so, they served for 12 years, followed by thirteen full years of rebellion. The war followed in the fourteenth year since the rebellion began. 1
Rashi writes: "'They served' - [i.e.] the five kings served." How might we have misinterpreted the verse?
Gur Aryeh: We should not think that all of the kings who gathered at Emek ha'Sidim joined the rebellion against Kedorla'omer, because that would include Amrafel, Aryoch and Tid'al (who remained his allies). Rather, only the five latter kings rebelled.