Passage: The Roman Empire reached its greatest territorial extent during the 2nd century AD; the following two centuries witnessed the slow decline of Roman control over its outlying territories. Economic issues, including inflation, and external pressure on the frontiers combined to create the Crisis of the Third Century, with emperors coming to the throne only to be rapidly replaced by new usurpers. Military expenses increased steadily during the 3rd century, mainly in response to the war with the Sasanian Empire, which revived in the middle of the 3rd century. The army doubled in size, and cavalry and smaller units replaced the Roman legion as the main tactical unit. The need for revenue led to increased taxes and a decline in numbers of the curial, or landowning, class, and decreasing numbers of them willing to shoulder the burdens of holding office in their native towns. More bureaucrats were needed in the central administration to deal with the needs of the army, which led to complaints from civilians that there were more tax-collectors in the empire than tax-payers.The Emperor Diocletian (r. 284–305) split the empire into separately administered eastern and western halves in 286; the empire was not considered divided by its inhabitants or rulers, as legal and administrative promulgations in one division were considered valid in the other. In 330, after a period of civil war, Constantine the Great (r. 306–337) refounded the city of Byzantium as the newly renamed eastern capital, Constantinople. Diocletian's reforms strengthened the governmental bureaucracy, reformed taxation, and strengthened the army, which bought the empire time but did not resolve the problems it was facing: excessive taxation, a declining birthrate, and pressures on its frontiers, among others. Civil war between rival emperors became common in the middle of the 4th century, diverting soldiers from the empire's frontier forces and allowing invaders to encroach. For much of the 4th century, Roman society stabilised in a new form that differed...

Question: at what tinme did the civil war between the emprors became more common?
middle of the 4th century,