(O ye who believe!) He means the Companions of Muhammad (pbuh) (When you meet an army) a group of the disbelievers on the Day of Badr, (hold firm) along with your prophet in war (and think of Allah much) in your hearts and with your tongues, saying: “there is no deity except Allah”, and “Allah is the greatest”, (that ye may be successful) that you may be safe from Allah's wrath and punishment and be assisted.