 Complete the following phrase:  Political instability in Rome grew. Emperor Caligula 37-41 was murdered by the Praetorian Guard to replace him with Claudius 41-53, while his successor Nero 54-68 burned Rome down. The average reign from his death to Philip the Arab 244-249 was six years. Nevertheless, external expansion continued, with Trajan 98-117 invading Dacia, Parthia and Arabia. Its only formidable enemy was the Parthian Empire. Migrating peoples started exerting pressure on the borders of the empire. The drying climate of Central Asia forced the Huns to move, and in 370 they crossed Don and soon after the Danube, forcing the Goths on the move, which in turn caused other Germanic tribes to overrun Roman borders. In 293, Diocletian 284-305 appointed three rulers for different parts of the empire. It was formally divided in 395 by Theodosius I 379-395 into the Western Roman and Byzantine Empires. In 406 the northern border of the former was overrun by the Alemanni, Vandals and Suebi invaded. In 408 the Visigoths invaded Italy and then 