Emperor Septimius Severus (193-211) is known for having one of the most successful reigns. However, he raised taxes and expanded the army, making it compulsory to be in the military before holding political office. Septimius’s son Carcalla took the throne after murdering his brother. Following in his father’s footsteps, Carcalla increased the military’s pay by 50% and extended citizenship to the entire empire. 

During the third century, tribes of Germanic barbarians on the northern border of the Empire started penetrating Roman defenses. The Roman economy had grown reliant on conquest and pillage of other nations, so when the invasions stopped, most of the money dried up, which caused taxes to be raised. From continual wars and diseases, the Roman population started to decline rapidly. 

The next 15 emperors after Carcalla were all either killed by the Praetorian Guard, in battle, or in a conspiracy, except for Emperor Carus, who was struck by a bolt of lightning. In 280, Rome made a truce with Persia to focus more military spending on the barbarians, and in 284, Diocletian became emperor after a coup. 

Emperor Diocletian moved the capital from Rome to Nicomedia in the east, because that was the wealthier side of the empire. Later, he divided the empire into two sections (East and West). These changes helped the empire some, but Diocletian also set prices for food, making it illegal for store owners to charge more than a certain price, and outlawed unemployment. 

Although Diocletian’s reign was somewhat more stable than emperors before him, the Empire fell into infighting again, eventually resulting in Constantine taking the throne and legalizing Christianity. Constantine made more reforms and moved the Eastern capital to Constantinople. 

In the 4th century, the Huns, a nomadic tribe from western Asia, fought the Visigoths, a Germanic tribe who lived on the northern frontier. Roman Emperor Valens allowed the Visigoths to enter the Empire to protect them against the Huns. However, in 378 the Visigoths rebelled against Rome and defeated Valens at the Battle of Adrianople. The Visigoths then plundered and burned Thrace, murdering the population. 

In 395, the Visigoths destroyed most of the Italian peninsula, which caused Roman forces to abandon Britain and the northern border to go south. Without border protection, hordes of barbarians poured over the border. In 410, Rome was sacked by barbarians, who rampaged the city for three days and killed or enslaved most of the population. Shortly after, Gaul and Spain fell to Vandals, leading to the complete collapse of the Western Roman Empire. 

The Western Empire officially dissolved in 476, whereas the East continued. The Eastern Empire was renamed the Byzantine Empire and would continue to exist until the 15th century, eventually falling to the Ottoman Empire.