Vasudeva had no weapon with him. He tried to save his new bride, but he had only his wits to use. He first tried to convince Kamsa by good logic and philosophy, telling him that we are not these bodies, and that the soul never dies. But Kamsa did not care, so the only thing left for Vasudeva to do was to promise to hand over the babies as they were born. He was thinking, by then I will figure out something, but at least Devaki will be saved. Kamsa accepted this because he knew Vasudeva was always true to his word.
