Or HaChaim on Exodus 14:14/אור החיים על שמות י״ד:י״ד

1 ה׳ ילחם לכם, "G'd will fight on your behalf, etc." Inasmuch as the reason the Israelites had been frightened had been that they saw themselves confronting celestial forces, G'd tells them that the most powerful celestial force, He Himself, will fight on their behalf. When G'd is involved personally, even a thousand celestial forces equal to the guardian angel of Egypt are nothing to be afraid of.

2By emphasising the attribute השם i.e. the attribute of Mercy, the Torah suggests that even the attribute of Mercy concurred with the retribution G'd was about to exact from the Egyptians. The Torah writes ילחם לכם, "He will fight on your behalf," because from Israel's point of view this would be a manifestation of the attribute of Mercy. Moreover, the word ילחם implies much more than mere assistance. G'd was going to conduct the entire war single-handedly, hence ואתם תחרישו "you have to keep silent."

3We find a comment in the introduction to Eychah Rabbati 30 according to which four different righteous people when in difficulties each asked something different from G'd. The one who represented the highest level of righteousness refrained from asking at all. The model for this approach quoted is King Chizkiyah who said (when facing the onslaught of Sancheriv): "I neither possess the strength to kill, nor to pursue, nor even to recite hymns of praise; hence I will sleep on my bed and You G'd will do what needs to be done." G'd did indeed kill the army of Sancheriv without any involvement of King Chizkiyah or his forces (Kings II chapter 19). When Moses told the people that all they had to contribute was their silence, he implied that their present state of righteousness was of the same calibre as that of King Chizkiyah. According to the interpretation we offered that the word התיצבו means to stand still in prayer, we have to understand the words: "but you shall remain silent," as addressed to the attribute of Justice. The Israelites were to offer prayers in order to silence the attribute of Justice which might otherwise appeal to G'd claiming that they were not worthy of the miracle about to be performed on their behalf.