Genesis 49:19

Gad, a troop shall overcome him— Jacob, alluding to the name of Gad, which signifies a troop, foretels that this tribe should have many enemies to struggle with, who should sometimes get the better of them; but that in time they, by the Divine aid, should prove victorious over all opposition. Durell renders it, A troop shall invade Gad, but he shall invade their rear. This part of the prophecy, he observes, seems to have been fulfilled the soonest of any: Sihon, the king of the Amorites, refused the Israelites a free passage through his country to the land of Canaan; and, not content with this, levied a large army, and, in conjunction with Og the king of Bashan, marched out, and attacked the Israelites. The consequence of this rash expedition was, that both those kings and their armies met with a total overthrow, and lost their country and all that they had. The country of the Amorites was given by Moses to the Gadites, probably because they had been chiefly instrumental in subduing it; for they are frequently represented as some of the bravest soldiers of all Israel; and it is with reference to that grant, that what he says of this tribe, in the parallel place, Deuteronomy 33:21 is to be understood; and I think it is so here.