17. You killed them not, but Allah killed them. And you threw not when you did threw, but Allah threw, that He might test the believers by a fair trial from Him. Verily, Allah is All-Hearer, All-Knower.

Allah states that it was not by their power and strength that they killed the pagans, who were numerous and they were few. Rather, it is He Who gave them victory over them, just as He said in another Ayah:
<And Allah has already made you victorious at Badr, when you were a weak little force.>
Allah says, "It was not you who killed them, but Allah Who killed them." It was said that this Ayah was revealed about the angels' killing the idolaters on the day of Badr, or about a handful of sand the Messenger of Allah threw at the idolaters when he said: "Humiliated be their faces." He invoked Allah humbly and expressing his neediness before Allah. He then threw a handful of sand at the disbelievers and said,
«Humiliated be their faces.» He then commanded his Companions to start fighting with sincerity and they did. Allah made this handful of sand enter the eyes of the idolaters, each one of them were struck by some of it and it distracted them making each of them busy. Allah said,
<And you threw not when you did throw, but Allah threw.>
Therefore, it is Allah Who made the sand reach their eyes and busied them with it, not you (O Muhammad).
Muhammad bin Ishaq said that Muhammad bin Ja'far bin Az-Zubayr narrated to him that 'Urwah bin Az-Zubayr said about Allah's statement,
<that He might test the believers by a fair trial from Him.>
"So that the believers know Allah's favor for them by giving them victory over their enemy, even though their enemy was numerous, while they were few. They should thus know His right and express gratitude for His favor on them." Similar was said by Ibn Jarir. It is stated in a Hadith,
«Every trail (from Allah) is a favor for us.»
Allah said next,
<Verily, Allah is All-Hearer, All-Knower.>
Allah hears the supplication and knows those who deserve help and triumph. Allah said,
<18. This (is the fact) and surely Allah weakens the deceitful plots of the disbelievers.>
This is more good news, aside from the victory that the believers gained. Allah informed them that He will weaken the plots of the disbelievers in the future, degrade them and make everything they have perish and be destroyed, all praise and thanks are due to Allah.