Overall meaning : Allah directs His Prophet, Muhammad, may Allah bless him and grant him peace, to inform the Quraysh unbelievers that if they desist from unbelief and from waging war on the believers, Allah will forgive them their past sins, but if they return to fighting the believers and persisting in their unbelief, then His way of destroying the Quraysh unbelievers at the Battle of Badr, as well as His way of dealing with unbelieving nations in general, has already gone before. They have already before them the doom of the ancients and they could suffer the same terrible fate.
He also commands His believing servants to continue to fight against the unbelievers if they persist in their unbelief and hostilities until there is no more idolatry and unbelief on earth and all worship is devoted to Allah alone. If they desist, then Allah sees all that they do, but if they pay no heed and persist in their unbelief and hostilities, then the believers should know for certain that Allah is their protector and master, who will definitely support them against them—an excellent protector and an excellent helper.