Now when Pharaoh let the people go, God did not lead them by way of the land of the Philistines, although it was nearer; for God said, “The people may have a change of heart when they see war, and return to Egypt.”

ויהי בשלח פרעה... ולא נחם AND IT CAME TO PASS WHEN PHARAOH HAD SENT [THE PEOPLE AWAY] THAT GOD GUIDED THEM NOT — The word נחם means He guided them, just as, (Exodus 32:34) “Go, guide (נחה) the people”, and (Proverbs 6:22) “when thou goest it shall guide (תנחה) thee”.

כי קרוב הוא BECAUSE IT WAS NEAR, and it would therefore be easy to return by the same route to Egypt. — Of Midrashic explanations there are many (cf. Mekhilta d'Rabbi Yishmael 13:17).

בראתם מלחמה WHEN THEY SEE WAR — For instance the war mentioned in (Numbers 14:45) “Then the Amalekites and the Canaanites came down etc.” If they had proceeded by the direct route they would have then turned back. This is evident, for what would have been the case? If, when He led them about by a circuitous way, they said, (Numbers 14:4) “Let us appoint another chief and go back to Egypt”, had He led them by a direct route how much the more certainly would they have spoken so.

פן ינחם PERADVENTURE [THE PEOPLE] REPENT — peradventure they cherish a different thought (they change their mind) about having gone out and set their hearts on returning (cf. Rashi on Genesis 6:6).

So God led the people round about, by way of the wilderness at the Sea of Reeds.
Now the Israelites went up armed out of the land of Egypt.

ויסב means HE LED THE PEOPLE ABOUT from the direct route to a circuitous route.

ים סוף is the same as לים סוף TO THE RED SEA. The word סוף has the meaning of a marshy tract in which reeds grow; examples are: (Exodus 2:3) “She placed him in the reeds (בסוף)”; (Isaiah 19:6) “The flags and the reeds (וסוף) shall wither”.

וחמשים — The word חמשים means provided with weapons (Mekhilta d'Rabbi Yishmael 13:19:2). — [Because He led them by a circuitous route through the wilderness He brought it about that they went up from Egypt well-provided; for had He led them circuitously by the route of an inhabited district they would not have provided for themselves every thing that they needed, but only a part, like a person who is travelling from place to place and intends to purchase there whatever he will require. But if he were setting out for the wilderness he must provide all that he will require. — This verse (statement in the verse) is written only with the view of making the ear understand (preparing you for a later statement) viz., that you should not wonder with regard to the war with Amalek and the war with Sihon and Og and Midian where they obtained weapons, since they smote them with the sword]. In a similar sense it says, (Joshua 1:14) “and ye shall pass over armed (חמשים)”. Onkelos, too, translated it by מזרזין which signifies “armed” in Aramaic, just as he translates the word וירק in (Genesis 14:14) וירק את חניכיו which means, “And he armed his trained servants” by וזריז. Another explanation of חמשים is: only one out of five (חמשה) went forth from Egypt, and four parts of the people died during the three days of darkness because they were unworthy of being delivered (Mekhilta d'Rabbi Yishmael 13:19:3; cf. Rashi on 10:22).