Yisro heard about two events: the miracles at the Red Sea, when the Egyptians were punished for their treachery; and the war against Amalek, when Klal Yisrael triumphed over their archenemy. Yisro was not the only one who “heard.” Many heard; he, however, internalized it and acted positively in response. Why did Yisro need two incidents to impress upon him the greatness of Hashem and His People? Was not the splitting of the Red Sea a sufficient miracle to influence his way of thinking? Indeed, the war with Amalek could have been misconstrued as a victory effected by Klal Yisrael’s military genius. The splitting of the Red Sea, however, was a miracle of the highest order. No one could question it. Why, then, was it necessary for Yisro to hear also about the war with Amalek before he decided to join Klal Yisrael?
Horav Shlomo Gestetner, Shlita, explains that while Krias Yam Suf was an incredible miracle that had no equal, one might err and think that Hashem produced this miracle in order to save Klal Yisrael whose lives were in grave danger. Indeed, if another nation had been pursued by Egypt, Hashem would likely also have saved them from their oppressors. In other words, Krias Yam Suf does not demonstrate a clear indication that Hashem sought to save Klal Yisrael specifically because they would soon receive the Torah and mitzvos. Only after Yisro saw that they were rescued from Amalek, whose sole desire was to destroy them spiritually, was he convinced that Klal Yisrael’s spiritual destiny was what mattered to Hashem. Their spiritual ascendancy gave the Jewish People preeminence over the other nations. This motivated Yisro to leave Midyan and join the Jews.