At the end of the previous surah (al Kaafiruun), the prophet was instructed to say to the negotiators of the disbelievers: "To you your religion, and to me my religion." In this Madani surah, Allah is telling His messenger which of the two parties will get the upper hand. Seeing people enter the fold of Islam in multitude is a natural consequence of the prophet's victory over his enemies, who used to be a big obstacle preventing people from seeing the truth of Islam.
The surah reminds us that victory is something that belongs to Allah alone and He chooses when, where, and how to send it, and if He sends it to us we should exalt Him, praise Him, and ask for His forgiveness. Asking for forgiveness is especially important because after achieving a victory that appears to be a result of our effort, we tend to forget that we are nothing more than tools in the hands of Allah, who controls the situation.
The last ayah teaches us some of the etiquette of dua. First we exalt Allah, then we praise Him, and finally we ask Him for what we need.