The Civil Wars of the Tetrarchy were a series of conflicts between the co-emperors of the Roman Empire, starting in 306 AD with the usurpation of Maxentius and the defeat of Severus, and ending with the defeat of Licinius at the hands of Constantine I in 324 AD
The Tetrarchy refers to the administrative division of the Roman Empire instituted by Roman Emperor Diocletian in 293 CE, marking the end of the Crisis of the Third Century and the recovery of the Roman Empire. The first phase, sometimes referred to as the Diarchy ('the rule of two'), involved the designation of the general Maximian as co-emperor - firstly as Caesar (junior emperor) in 285, followed by his promotion to Augustus in 286. Diocletian took care of matters in the Eastern regions of the Empire while Maximian similarly took charge of the Western regions. In 293, feeling more focus was needed on both civic and military problems, Diocletian, with Maximian's consent, expanded the imperial college by appointing two Caesars (one responsible to each Augustus) - Galerius and Constantius Chlorus.
The senior emperors jointly abdicated and retired in 305 AD, allowing Constantius and Galerius to be elevated in rank to Augusti. They in turn appointed two new Caesars - Severus in the west under Constantius, and Maximinus in the east under Galerius.