The Crisis of the Third Century lasted from 235 CE until the reforms of Diocletian, almost fifty years later. During the crisis, Roman territory was split into three competing empires: The Gallic Empire in modern-day France, Iberia, and Britain; the Palmyrene Empire in modern-day Syria, Egypt, and Asia Minor; and the remainder of the Roman Empire, which included Italy, Greece, and North Africa. Aurelian, who ruled from 270 to 275 CE, conquered the Gallic and Palmyrene empires and reunited the Roman Empire.  

The Crisis of the Third Century began in 235 CE and lasted for fifty years. It began with the assassination of Severus Alexander (the last ruler of the Severan Dynasty). Severus was assassinated by his own troops for engaging in diplomacy with invading barbarian peoples (the troops felt this was a sign of weakness). The Crisis of the Third Century was a chaotic and destructive time in Roman history, as many different factions competed for ultimate control of the empire.

The Crisis of the Third Century was primarily caused by the growing threats to the Roman Empire from outside the empire’s borders. Climate change in northern Europe led to a mass migration of barbarian people into Roman lands — these people were not conquerors, but were merely looking for a place to settle and farm. Nonetheless, it exacerbated economic tensions in Roman society. Additionally, Rome was dealing with more and more attacks along the eastern frontiers, as warring tribes from eastern Europe continuously attacked and pillaged Roman border communities. Furthermore, the growing power and influence of the Sassanid Empire (in the Middle East) made it more expensive for Rome to hold on to its Asiatic holdings. Finally, the lack of a clear line of succession in Roman society had long been a major problem. Because there was no clear and absolute way to choose a new Emperor, it was common for competing claims to be made to the throne whenever an Emperor died. This led to civil wars throughout the history of the Roman Empire, but none were as prolonged or as devastating as the Crisis of the Third Century.

The Battle of Edessa was fought in the third century between the armies of the Roman Empire, led by Emperor Valerian, and the armies of the Sassanid Empire. It ended in absolute victory for the Sassanid forces and the capture of the Roman Emperor.

The Battle of Margus was fought in 284 CE between the forces of the usurper Diocletian and the forces of the Emperor Carinus. It ended in victory for Diocletian, who subsequently took full control of the Roman Empire. Diocletian then enacted a series of reforms that brought to an end the Crisis of the Third Century.

In the third century CE, the Roman Empire underwent a period of fifty years in which power was heavily decentralized. During this period, there were countless claimants to the position of emperor and no political uniformity in the Roman Empire. For a brief period, the Roman Empire was divided into three territories, each ruled by a different emperor. Eventually, the Roman Empire was stabilized under the rule of Aurelian. The Crisis of the Third Century came to an end shortly thereafter with a series of reforms made by the Emperor Diocletian.

In 235 CE the Roman army was, indeed, on the ropes from Germanic invasions in the North, but the most accurate answer here is the assassination of Severus Alexander by his own troops. The military problems in the North had persisted for sometime, but the real internal strife of the crisis began when the ruler of the Empire was murdered by his own troops as he tried to pacify the Germanic tribes.