Excerpts from Books and Wikipedia
"The Edict of Toleration by Galerius was issued in 311 by the Roman Tetrarchy of Galerius, Constantine and Licinius, officially ending the Diocletian persecution of Christianity." [Wikipedia]
"During his reign, he campaigned, aided by Diocletian, against the Sassanid Empire, sacking their capital Ctesiphon in 299. He also campaigned across the Danube against the Carpi, defeating them in 297 and 300. Although he was a staunch opponent of Christianity, Galerius ended the Diocletianic Persecution when he issued an Edict of Toleration in Serdica in 311." [Wikipedia]