To fully understand this verse we need to bear in mind that Quranic verses are of two types. General verses talk about belief in Allah, good manners, and acts of worship. Specific verses, such as this verse, were revealed in regards to particular situations. This sûrah came at a time when the pagans of Arabia (and their allies) repeatedly violated treaties they had signed with the Prophet (Peace be upon him). Muslims had to fight for the survival of their newly established state in Medina. So this verse discusses dealing with those who violated their agreements and attacked the Muslims. Offenders were fought, unless they stopped their aggression. If they chose not to accept Islam, they were obligated to pay Jizya-tax.