"Then We fulfilled the promise to them." The messengers used to warn their people of the punishment of Allah in this world and the Hereafter if they did not believe. When they did not believe, Allah fulfilled His promise to His messengers, and He sent down the punishment upon their people.
He said: "And We saved them and whom We willed," meaning: the prophet and the believers, "and We destroyed the transgressors," [meaning] the polytheists.