Maximian played a key role in Diocletian's tetrarchy. Diocletian was the brains while Maximian provided the muscle. The men became co-emperors under Diocletian's reforms. Diocletian was warrior, politician, and political theorist. On the other hand, Maximian was a soldier. Maximian used his martial abilities and close ties to Diocletian to rise to power.
The future Augustus Maximian was born in the war torn Pannonia Province in modern Serbia. The harsh frontier conditions translated to a military career. The legions provided a possible escape for the shopkeep's son. Maximian met Diocletian while serving under the Emperor Aurelian. The two men developed a long term alliance and possibly a friendship over the next decade.
The pair served under Aurelian, Probus, and probably Carus from 270-283. Maximian developed a reputation as a solid warrior and soldier. He seems to have been a leader of men and above average general. Most importantly, the aggressive Maximian followed orders. Unlike many Roman generals, he did not seem to want to seize power. He wanted to be a soldier and not a politician while Diocletian was a political animal.
Diocletian rose to the imperial purple in late 284. Shortly thereafter, the emperor appointed Maximian his Caesar, or co-emperor. With conflicts raging throughout the empire, Diocletian needed a skilled soldier he could trust. On top of this, Diocletian had no sons to found a dynasty. Maximian may have been adopted by Diocletian to solidify succession. Afterward, the Caesar marched west to fight Rome's enemies. Maximian fought rebels and barbarians during his tenure. In the end, he helped Diocletian pacify the empire and restore the peace.
Maximian rode Diocletian's friendship to power. However, his character proved as important as his martial skill and relationships. Maximian was a loyal fighting general disinterested in politics. This made him the perfect co-emperor for Diocletian. Maximian freed Diocletain to reform the empire.