Or HaChaim on Exodus 13:18/אור החיים על שמות י״ג:י״ח

1 וחמושים עלו בני ישראל, and the Israelites went up armed, etc. It is possible that if the Israelites had not been armed they would not have listened to Moses' (G'd's) order to make an about turn away from the land of the Philistines so that they would not face an immediate battle. The Israelites therefore needed two reasons for not returning to Egypt at the slightest danger, i.e. possession of arms and the unlikelihood of having to use these arms. Combining these two factors would result in לא ינחם, that they would not have second thoughts.