Or HaChaim on Exodus 15:18/אור החיים על שמות ט״ו:י״ח

1 ה׳ ימלך לעולם ועד, "The Lord will rule for ever and ever." The author implies that the reason he did not speak about the Lord having reigned in the past is because of the challenge to G'd's rule described in verse 19. Inasmuch as G'd has exacted retribution from only one nation which has not recognised His sovereignty in the world it would be inappropriate to extol G'd as having ruled in the past. כי בא סוס פרעה, "For the horses of Pharaoh came, etc." The fact that G'd disposed of the challenge represented by Pharaoh was not yet proof of the impotence of all the powers which compete with G'd on earth and which different nations worship. G'd's true sovereignty will be established only when He deals with all competing deities.

2Another way of understanding the sequence of the words in these two verses plus the fact that G'd is described as sovereign only in the future is this: "The Lord will rule only in the future because all of Pharaoh's horses and their riders perished as one man. As a result the only ones left to tell about G'd's mighty deeds were the Israelites who were saved in the midst of the sea." Had there been a few Egyptians who had saved themselves and those Egyptians had told the rest of the world of their experiences this would have suitably impressed the world. As it was, only the Israelites who had already recognised G'd's sovereignty were around to tell the tale. This was not enough to convince the pagans.