194-199              Septimius Severus twice campaigns against the Parthians

208-211              Septimius Severus campaigns against the Caledonians in northern Britain

211                     Death of Septimius Severus; accession of Caracalla; murder of Geta

212                     All free men of the empire granted Roman citizenship

217                     Death of Caracalla

218                     Accession of Elagabalus

222                     Death of Elagabalus; accession of Severus Alexander

235                     Death of Severus Alexander

235-284             The empire endures a period of crisis; no fewer than 18 emperors nominated by the Senate

260                     The Sassanids capture Antioch

260-274              Breakaway ‘Empire of the Gauls’ formed in Gaul and Britain

262-273              Breakaway Palmyrene kingdom in the eastern empire

270-275              Aurelian restores some order as emperor and builds new walls for Rome

284                      Accession of Diocletian

293                      Diocletian establishes the Tetrarchy