وَمَا جَعَلَهُ اللّهُ إِلاَّ بُشْرَى لَكُمْ {And Allah made it not except as [a sign of] good tidings for you}: That is, Allah (swt) made the angels descend upon you to fight with you just as good news for you because victory is from Allah (swt) alone, so he followed it by saying:
{وَمَا النَّصْرُ إِلاَّ مِنْ عِندِ اللّهِ الْعَزِيزِ الْحَكِيمِ} {And victory is not except from Allah, the Exalted in Might, the Wise}: Victory is not made by angels, so if Allah (swt) wants to help the believers, then He does not need to provide them with angels fighting on their side, Allah (swt) wanted to clarify this part. So sending down of angels is a tiding and reassurance for the companions that there are many to fight with them, whereas, the victory, was determined and it was from Allah (swt) and not from the angels and their fight with them.
That is why before the battle, when the Prophet (saws), spent the whole night of the seventeenth of Ramadan, the night of the Battle of Badr, supplicating to his Lord as ‘Umar bin Al-Khattab said:
“The Prophet (saws) looked over the idolaters, and there were a thousand of them, while his Companions were three-hundred and ten and some odd number of men. So the Prophet of Allah (saws) faced the Qiblah, stretched forth his hands and began beseeching his Lord: ‘O Allah! Fulfill what You promised for me. [O Allah! Bring about what You promised for me] O Allah! If you destroy this band of adherents to Islam, you will not be worshiped upon the earth,’
He kept beseeched his Lord with his hands stretched, facing the Qiblah until his robe fell from his shoulders. Abu Bakr, then, came to him, took his robe and placed it back upon his shoulders, then embraced him from behind and said:
‘O Prophet of Allah! You have sufficiently beseeched your Lord, indeed He shall fulfill what He promised you.
The Prophet (saws) used to supplicate to Allah (swt) while he was certain of the answer, and he was also certain of victory; because it was revealed to him before the Battle of Badr, Allah said:
سَيُهْزَمُ الْجَمْعُ وَيُوَلُّونَ الدُّبُرَ
{Their] assembly will be defeated, and they will turn their backs [in retreat].} (Al-Qamar: 45)
Someone may say: As long as he (saws) is certain, why did he pray all night? He (saws) paid the price of victory in advance, so the supplication is either for a need or it is a worship. So that the angels descending was a reassurance for the Companions during the fighting but they are not the ones who achieved victory, the victory is from Allah (swt), The Mighty, The Wise, as He said: {And victory is not except from Allah, the Exalted in Might, the Wise}.
Why didn’t Allah (swt) say: The Compeller and the Avenger? Why did he not say: The Strong, the All-Knowing?
In the Holy Qur’an, every word attracts its meaning, the attributes of Allah (swt) come in accordance to the events that mentioned in the verse. That is, Allah (swt), is Exalted in Might because He is invincible, and He is The Wise because He wills the victory at the suitable time that He knows.