משך חכמה — ה' ילחם לכם ואתם תחרישון

Q — Why does the Mechilta read "לא לשעה זו בלבד" — extending the promise to all future generations, when on the simple reading it addresses the moment at the Sea?

A — At the Sea Israel had a fully legitimate claim — "why did you bring us out to die?" — so Hashem was bound by din to fight for them; the chiddush of "ואתם תחרישון" is its application to FUTURE times, when Israel will have nothing to say in their own defense ("בושת הפנים לדבר") — and even then, with no merit and no argument, Hashem will still fight for them.