Why did Diocletian choose Maximian as his co-emperor?

Diocletian chose to share the throne with Maximian primarily because the Roman Empire had become too vast and unstable for a single ruler to manage effectively. During this period, the empire faced constant threats, including frequent internal revolts and external invasions along its extensive frontiers, ranging from Britain to the Persian Gulf.1

Diocletian, who preferred to focus on administration and governance, required a trusted partner who was both a skilled soldier and a faithful companion to take primary responsibility for military defense. Maximian, an Illyrian who had risen through the army ranks based on his military prowess, was a trusted officer and friend of Diocletian. By appointing Maximian in 286, Diocletian was able to delegate the military defense of the West—specifically Italy, Spain, and Africa—to him, while Diocletian focused on the East. This decision marked the beginning of a collaborative system of rule that eventually evolved into the tetrarchy, allowing for more efficient administration and a quicker response to threats across the empire's diverse regions.123