What is the donatist controversy?

An event that took place in the 4/5th century in Rome. Some Christians bent to the will of Diocletian during the great persecution and the Donatists didn't think that those people's faith was valid any longer. There was controversy over who should be bishop of Carthage. So they sent representatives to Rome to argue their case but Constantine ruled that they were wrong. They still wouldn't give up so Constantine persecuted them, but that just made them martyrs.