During the following years, the empire saw barbarian invasions and migrations into Roman territory, civil wars, peasant rebellions and political instability, with multiple usurpers competing for power.

Aurelian successfully reunited the empire by defeating the two breakaway states, and carried out a series of reforms which helped restore some measure of stability to the Roman economy.

[6] His son Caracalla raised the annual pay and lavished many benefits on the army in accordance with the advice of his father to keep their loyalty,[7][8][9] and considered dividing the empire into eastern and western sectors with his brother Geta to reduce the conflict in their co-rule.

This cut off the essential source of income gained from plundering enemy countries, while opening up the Roman countryside to economic devastation from looters both foreign and domestic.

Many Roman legions had been defeated during a previous campaign against Germanic peoples raiding across the borders, while the emperor Severus Alexander had been focused primarily on the dangers from the Sassanid Empire.

Leading his troops personally, the emperor resorted to diplomacy and accepting tribute to pacify the Germanic chieftains quickly, rather than military conquest.

According to Herodian, this cost Severus Alexander the respect of his troops, who may have felt that more severe punishment was required for the tribes that had intruded on Rome's territory.

Maximinus was the first of the barracks emperors – rulers who were elevated by the troops without having any political experience, a supporting faction, distinguished ancestors, or a hereditary claim to the imperial throne.

There were frequent raids across the Rhine and Danube frontier by foreign tribes, including the Carpians, Goths, Vandals, and Alamanni, and attacks from Sassanids in the east.

Climate changes and a sea level rise disrupted the agriculture of what is now the Low Countries, forcing tribes residing in the region to migrate into Roman lands.

[21] An invasion of Macedonia and Greece by Goths, who had been displaced from their lands on the Black Sea, was defeated by emperor Claudius II Gothicus at the Battle of Naissus in 268 or 269.

The right of imperial succession had never been clearly defined, which was a factor in the continuous civil wars as competing factions in the military, Senate, and other parties put forward their favored candidate for emperor.

The sheer size of the empire, which had been an issue since the late Roman Republic three centuries earlier, continued to make it difficult for a single ruler to effectively counter multiple threats at the same time.

[26] During the early Principate, the process for becoming an emperor relied on a combination of proclamation by the Senate, popular approval, and acceptance by the army, in particular the Praetorian Guard.

Whenever the succession appeared uncertain, there was an incentive for any general with support of a sizable army to attempt to seize power, sparking civil war.

After the overthrow of the Severan dynasty, for the rest of the 3rd century, Rome was ruled by a series of generals, coming into power through frequent civil wars which devastated the empire.

[29] These plagues greatly hindered the Roman Empire's ability to ward off barbarian invasions but also factored into problems such as famine, with many farms becoming abandoned and unproductive.

This could also have contributed to the increased barbarian pressure on Roman borders, as they too would have experienced the detrimental effects of climate change and sought to push inward to more productive areas of the Mediterranean region.

Other tribes coalesced into more formidable entities (notably the Alamanni and Franks), or were pushed out of their former territories by more dangerous peoples such as the Sarmatians (the Huns did not appear west of the Volga for another century).

[33] The Sassanids had in 224 and 226 overthrown the Parthian Arsacids, and the Persian King Ardashir I, who also wanted to prove his legitimacy through military successes, had already penetrated into Roman territory at the time of Severus Alexander, probably taking the strategically important cities of Nisibis and Carrhae in 235/236.

Large estates produced cash crops for export and used the resulting revenues to import food and urban manufactured goods.

The historian Henry St. Lawrence Beaufort Moss describes the situation as it stood before the crisis: Along these roads passed an ever-increasing traffic, not only of troops and officials but of traders, merchandise and even tourists.

Metals mined in the uplands of Western Europe, hides, fleeces, and livestock from the pastoral districts of Britain, Spain, and the shores of the Black Sea, wine and oil from Provence and Aquitaine, timber, pitch and wax from South Russia and northern Anatolia, dried fruits from Syria, marble from the Aegean coasts, and – most important of all – grain from the wheat-growing districts of North Africa, Egypt, and the Danube Valley for the needs of the great cities; all these commodities, under the influence of a highly organized system of transport and marketing, moved freely from one corner of the Empire to the other.

[38] Large landowners, no longer able to successfully export their crops over long distances, began producing food for subsistence and local barter.

[39] Made desperate by economic necessity, many of these former city dwellers, as well as many small farmers, were forced to give up hard-earned basic civil rights in order to receive protection from large land-holders.

Large landowners, who had become more self-sufficient, became less mindful of Rome's central authority, particularly in the Western Empire, and were downright hostile toward its tax collectors.

The common people of the empire lost economic and political status to the land-holding nobility, and the commercial middle classes waned along with their trade-derived livelihoods.

The Crisis of the Third Century thus marked the beginning of a long gradual process that would transform the ancient world of classical antiquity into the medieval one of the Early Middle Ages.

Hence, the balance of power clearly shifted eastward during this period, as evidenced by the choice of Diocletian to rule from Nicomedia in Asia Minor, putting his second in command, Maximian, in Milan.

The Senate ceased to be the main governing organ and instead members of the equestrian class who filled the military officer corps became increasingly prominent.