Allah said,

وَمَا جَعَلَهُ اللّهُ إِلاَّ بُشْرَى لَكُمْ وَلِتَطْمَئِنَّ قُلُوبُكُم بِهِ ...

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,

ذلِكَ وَلَوْ يَشَآء اللَّهُ لاَنْتَصَرَ مِنْهُمْ وَلَـكِن لّيَبْلُوَ بَعْضَكُمْ بِبَعْضٍ وَالَّذِينَ قُتِلُواْ فِى سَبِيلِ اللَّهِ فَلَن يُضِلَّ أَعْمَـلَهُمْ

سَيَهْدِيهِمْ وَيُصْلِحُ بَالَهُمْ

وَيُدْخِلُهُمُ الْجَنَّةَ عَرَّفَهَا لَهُمْ

But if it had been Allah's will, He Himself could certainly have punished them (without you). But (He lets you fight) in order to test some of you with others. But those who are killed in the way of Allah, He will never let their deeds be lost. He will guide them and set right their state. And admit them to Paradise which He has made known to them. (47:4-6)

This is why Allah said here,

وَمَا جَعَلَهُ اللّهُ إِلاَّ بُشْرَى لَكُمْ وَلِتَطْمَئِنَّ قُلُوبُكُم بِهِ وَمَا النَّصْرُ إِلاَّ مِنْ عِندِ اللّهِ الْعَزِيزِ الْحَكِيمِ ﴿١٢٦﴾

Allah made it not but as a message of good news for you and as an assurance to your hearts. And there is no victory except from Allah, the All-Mighty, the All-Wise.

This Ayah means,

"Allah is the Almighty Whose power can never be undermined, and He has the perfect wisdom in His decrees and in all His decisions.''