Overall meaning : Believers, if you aid the cause of Allah by fighting against His enemies, He will aid you against them and make you stand firm when engaging in battle with them. As for those who reject the truth, they will be utterly disgraced and destroyed, and He will render their deeds vain. They will incur such misery because they have rejected with abhorrence the Qur’an that Allah has revealed, and so He has made their deeds fruitless.

Have the Quraysh idolaters never journeyed through the land and seen the terrible fate of the unbelievers before them? Allah completely destroyed them, and the same dreadful end awaits the unbelievers if they do not believe.
Such support for the believers and destruction for the unbelievers will come to pass because Allah is the protector of the faithful, while the unbelievers have no protector to take care of them and help them.