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|King of Britannia|
|Reign||411 AD - 2 September 421 AD|
|Full name||FLAVIVS CONSTANTIVS AVGVSTVS|
|Died||2 September 421|
|Place of death||Londinium, Britannia|
|Consort to||Domita Aurelia (399 - 405)|
|Wife||Galla Marcia (418 - )|
|Royal House||House of Constantine|
Flavius Claudius Constantinus, known commonly as Constantine I (died 2 September 421 AD) was the first King of Britannia. He is the founder of the House of Constantine, and the father of Valentinian I and Maxentius I.
Constantine was born a Roman and served as a general for several years who governed the Roman territory of Britannia. In 406 AD several barbaric invasions accured in which the Vandals, Burgundians, Alans, and the Sueves enter Gaul, cutting off contact between the British Isles and Rome. At the time of this invasion, the provinces of Britain were in revolt, after the unsuccessful rules of several upsurpers, which ended with the control under Constantine early in 407. The people rallied under their new ruler, who led several campaigns through Gaul, gathering support from the locals within the European mainland. By May 408 much of Gaul and the Rhine frontier had been secured by Constantine's forces, occupying the cities such as Arles, where the locals paid tribute to the government in Britannia in exchange for aid.
Recognized for his leadership the Emperor Honorius sent aid to support to the effort in the north. An imperial army under Constantine's son, Constans was sent after the Visigoths under Alaric. After a long campaign Alaric was routed back east and Constans was killed.
With the island of Britannia now fearing for its safety and threatening to overturn all Roman magistrates, the army loyal to Constantine abandoned Gaul for the isles. By 411 AD the entirety of Roman garrison in Britannia had been called back to Londinium. The remaining territory in northern Gaul was abandoned and all contact from the Emperor Honorius was largely ignored.
In 411 AD Constantine was declared first king of Britannia, an independent nation from Rome. Constantine largely spend the first several years of his rule quelling rebellions and attacking raiding parties within his fledgling nation. With barbarian migration increasing, Britannia was largely unable to halt the invaders, but rather cling to small pockets of Roman society, most notably the city of Londinium. Constantine would lead several campaigns through the enemy-occupied wilderness between strongholds, fighting off several aggressive tribes. During his reign small, mostly privately owned trade was able to continue, but at high risk. This small, underground loose band of merchants and guilds are what spread the news of Britannia's independence. Towards the end of his reign Constantine, now in less desirable health, initiated trade among the cities of Gaul, now largely considered city states under loose confederation. The Empire, which still continued to operate in the region of Gaul largely discouraged trade in many places. Constantine would also remarry with a British woman, giving birth to Valentinian. He would also adopt a local general known as Maxentius who served as king after him.
Constantine I died peacefully of natural causes in Londinium on 2 September 421 AD. On his deathbed he met with his successor, Maxentius, and with his warlord son, Ambrosius Aurelianus, who would later lead successful campaigns throughout Saxon territory. Rule of Britannia passed on to Maxentius I, second king of Britannia.