And to Allāh belongs whatever is in the heavens and whatever is on the earth. He forgives whom He wills and punishes whom He wills. And Allāh is Forgiving and Merciful.
Allah said "Allah made it not but as a message of good news for you and as an assurance to your hearts". This Ayah means, "Allah sent down angels and told you about their descent to encourage you and to comfort and reassure your hearts. You should know that victory only comes from Allah and that if He willed, He would have defeated your enemy without you having to fight them." For instance, Allah said after commanding the believers to fight,
In the previous verses, the battle of Badr was cited in relation to the battle of Uhud. Briefly stated there was the unseen support from Allah Almighty given to Muslims. In the present verses, some details of that support have been mentioned. Also stated is the wisdom behind the sending of angels.
One may naturally ask a question here. When Allah Almighty has endowed His angels with such power that just one of them could over-turn an entire habitation, very much like what happened with the land of the people of Lut (علیہ السلام) which was overturned single-handedly by the angel Jibra'il (علیہ السلام) ، why was it necessary to send out an army of angels? In addition to that, given the introduction of angels onto the battlefield, the obvious result should have been the total annihilation of every single infidel.
Believers should never be upset at any lack of manpower or resources. When small in number, they should have the conviction that God will amply compensate for this by sending angels down to help them. When confronted with inadequate resources, they should trust God to create such situations as will compensate for this deficiency. Success depends more on patience and being God-fearing (taqwa) than on material resources. Those who fear and trust God can receive His divine succour in two ways: first, by His causing a section of the enemy to be brought to repentance and submission, thus reducing their number; and second, by His bringing about their outright defeat. The first kind of God’s succour comes by way of dawah work. God opens to the truth the hearts of those opponents, who have some receptivity in them, and thus they join the believers. In this way they weaken the enemy forces and strengthen the believers’ ranks. By the second method, God gives strength and courage to the believers and enables them to emerge victorious.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.