Oh you who believed! You remember the favors of Allah bestowed upon you when enemies came to you, then We sent upon them a storm wind and an army which you couldn’t see, and Allah is watching what you are doing.
During the fifth year of Hijra (Migration of Messenger Muhammed and his followers from Maccah to Madeena), an army containing about 10000 persons of Arabian tribes came towards Messenger and his companions. According to the opinion of Salmanul Farisi, a Persian, they made a trench for not to allow the enemy to Madeena. That is why that war is known as the ‘War of Trench’. By defending the enemy’s army, Messenger and his companions stayed near the trench for one month. At last Lord, Allah sent a storm wind which caused them to fly away from the enemy, and fearfully scatter the enemy from there. “Army which you couldn’t see” told in the verse is the angels. See explanation 3: 124-127; 8: 12.