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http://www.chabad.org/library/bible_cdo/aid/9874/showrashi/true armed: Heb. חִמֻשִׁים וַחִמֻשִׁים [in this context] can only mean “armed.” (Since He led them around in the desert [circuitously], He caused them to go up armed, for if He had led them around through civilization, they would not have [had to] provide for themselves with everything that they needed, but only [part,] like a person who travels from place to place and intends to purchase there whatever he will need. But if he travels a long distance into a desert, he must prepare all his necessities for himself. This verse was written only to clarify the matter, so you should not wonder where they got weapons in the war with Amalek and in the wars with Sihon and Og and Midian, for the Israelites smote them with the point of the sword.) [In an old Rashi]) And similarly [Scripture] says: “and you shall cross over armed (חִמֻשִׁים)” (Josh. 1:14). And so too Onkelos rendered מְזָרְזִין just as he rendered: “and he armed (וְזָרֵיז) his trained men” (Gen. 14:14). Another interpretation: חִמֻשִׁים means “divided by five,” [meaning] that one out of five (חִמִֹשָה) [Israelites] went out, and four fifths [lit., parts of the people] died during the three days of darkness [see Rashi on Exod. 10:22]. — [from Mechilta, Tanchuma, Beshallach 1]