The third century was a very chaotic one. From instability in Europe to the rise of a new dynasty in China, this century greatly changed the geopolitical stage, put the transition to the Late Antiquity in place.
The most important events of this century took place in the slowly weakening Roman Empire. From 235 until the Empire's final days, the Imperial Crisis would plague Rome. This was a period of economic depression, plague, civil war, and raids; specifically from the tribes and hordes in the east. From this, Rome would be split into three. The Gallic Empire would gain control over the provinces of Gaul, Britannia, and Hispania, while in the Syrian desert the Palmyrene Empire would be formed, controlling Syria, Palestine, and Egypt, as well as parts of Asia Minor. These events would lead to the downfall of Rome, and a resurgence of feudalism and tribalism within the European continent.
Meanwhile in Eastern Europe, the influential Goth dynasty would rise to power in Romania and the Ukraine. While they did own a small state in what is now the crossroads between Poland, Ukraine, and the Belarus, these years would mark their rise to relevancy within the European continent. After a short invasion of the Sarmatian Khanate, they would come to establish a new nation in eastern Europe, the Ostrogothic Kingdom. The neighboring Gothic tribe to the west, in Dacia, the Thervingi, would boldly invade Rome with Ostrogothic support, and would, surprisingly, come out on top, leaving them to inherit all of Dacia and establish the Visigothic Kingdom.
This era of Gothic power would also see the Ostrogothic-Bosporean War, another surprising Gothic victory. As Bosporous was a Roman vassal, they were involved as well. As a result of this war, Bosporous would be exiled from the Crimean peninsula, owning just a small section of land on the Sea of Azov. The Goths would also later find themselves aligned with numerous other tribes to the east of Rome, such as the Carpians and Vandals, helping them invade and pillage Roman border towns, and later, helping them establish permanent, non-tribal authorities in their own regions. The Gothic states would, near the close of the century, support their former adversary in Bosporous during the Bosporean Revolution, calling neighboring tribes into the war and, ultimately, creating a Goth-friendly state on the Sea of Azov and ending Roman authority in Crimea. All of these defeats would greatly humiliate the Roman Empire.
This century also saw the rise of a new dynasty in Persia. After the fall of Parthia, a new dynasty, the Sasanians took control. The Sasanians would go on to create a golden age in Persia, being the last great Persian empire before its fall to the Caliphate. During the Sasanians' reign, Persian culture expanded far beyond its territorial borders, influencing areas as far east as Pakistan to as far west as Spain. Much of what would later become known as Islamic culture in art, architecture, music, and other subjects were transferred from the Sasanians through the Muslim world.
- The Roman Empire is placed under pressure by the Alemanni, Francia, the Ostrogoths, the Quadi, and the Sasanians. The Imperial Crisis starts.
- March 18- Roman Emperor Alexander Severus and his mother Julia Mamaea are murdered by legionaries in what is modern-day Mainz. This ends the Severan dynasty.
- March 20- Maximus Thrax is proclaimed the new Roman emperor.
- September 28- Pope Pontian abdicates after being exiled to Sardinia by Maximus Thrax. He starts up persecution of Christians in Rome.
- Maximus launches campaigns against the Dacians and Sarmatians.
- The Roman senate appoints a twenty-man committee to coordinate operations against
- Maximus and Marcus Pupienus Africanus Maximus become Roman consuls.
- Emperor Maximus campaigns on the Danube and Rhine, defeating the Alemanni.
- Maximus Thrax is accepted by the Roman senate. He taxes the aristocracy heavily and creates enough hostility among them that they plot against him.
- Ardashir of Persia attacks the Roman province of Mesopotamia.
- Maximus campaigns against the Carpians on the Danube.
- Future Roman emperor Valeria becomes princeps senatus.
- The Colosseum is restored after being damaged.
- The Goths, coming through Ukraine, cross the Danube and devastate Rome up to the border with Anatolia.
- The silver content of the Roman denarius falls to 28% under Gordian III.
- March 22- Roman subjects in Africa against Maximus and elect their own emperor, Marcus Antonius Gordianus. This revolt is seen as inspiration for later revolts in the Roman-owned Mahgreb.
- April 12- Numidian forces loyal to Maximus invade Africa with support of Legio III Agusta. Gordian II is killed and after a siege, his father commits suicide by hanging himself with his belt.
- April 22- The Year of the Six Emperors starts. The Senate outlaws Maximus for his bloodthirsty campaigns and nominates two Senate members, Pupienus and Balbinus, to the throne.
- June 24- Maximus Thrax is killed during a battle in Aquileia, alongside his son Maximinus.
- July 29- The Praetorian Guard storms the Roman palace and captures Pupienus and Balbinus. They are dragged nude through the streets and executed. Gordian III, aged thirteen, becomes emperor with Timesitheus as his tutor and advisor.
- Minor campaigns occur on the border between Rome and the Ostrogoths on their border in Bessarabia, referred to as Tyras under Rome.
- Ardashir destroys the city of Hatra.
- The Roman Empire is threatened on several fronts. The Franks raid the Rhine frontier and more revolts occur in Africa.
- The Kushan Empire's process of gradual weakening starts with Kashmir declaring independence.
- Manichaeism is created when Mani, a young mystic of Ctesiphon, proclaims himself a prophet and preaches his doctrine throughout Persia.
- Shapur I succeeds his father as king of Persia.
- Shapur I annexes parts of the dying Kushan Empire.
- Emperor Gordian and his armies reach Antioch, ready to fight the Sasanians.
- Gordian III begins a campaign against Shapur's armies.
- Cities in Bosporean Crimea are evacuated as the territory falls under the control of the Ostrogoths.
- A Roman army under Timesitheus defeats the Persians at Resaema. Shapur is forced to flee to the Euphrates, giving up all of the territories he had conquered.
- February 11- Gordian III is murdered by soldiers in Zaitha, a city in Mesopotamia. Phillip the Arab becomes the new Emperor, with the honoristic Augustus. He nominates his son, Philippus, as heir to the Roman throne.
- The vassal kingdom of Osroeme is absorbed into the Roman Empire.
- The silver content of the denarius falls to 0.5 percent under Phillip, down from 28 percent under Gordian III.
- In Britain, many thousands of acres of modern-day Lincolnshire are devastated by a great flood.
- Lady Trieu, a female Vietnamese warrior, begins her three-year resistance against the invading Chinese. In our world, she is sometimes referred to as the Vietnamese Joan of Arc.
- Emperor Phillip the Arab fights the Germans along the Danube.
- Phillip Augustus and his ten-year old son Philippus become Roman consuls.
- The Goths once again invade Rome from Ukraine and Romania on the Danube.
- Trajan Decius puts down a revolt in Moesia and Pannonia. At a battle in Verona, he defeats and kills current Emperor Phillip Augustus.
- A group of Franks invade the Roman Empire. Their armies penetrate as far as modern-day Tarragona in Spain.
- The Goths invade Rome, defeating their armies in the Battle of Augusta Traiana and laying siege to the cities of Plovdiv, Novae and Marcianopolis.
- Axum exerts their dominance of commerce on the Red Sea.
- An epidemic of smallpox begins in Ethiopia, moves into Egypt, and from there spreads to the Maghreb and the Roman holdings in Africa. It is named the Plague of Cyprian after St. Cyprian, bishop of Carthage.
- The Alemanni tribe invades Roman border towns, annexing the region and forcing the citizens to flee to safer territory.
- The Goths kill Roman Emperor Trajan Decius, at the Battle of Abritus. Hostilian, son of Trajan Decius, rises to the Roman throne but dies of smallpox in the same year.
- The prosperity of Britain declines as Germanic tribes such as the Franks and Saxons make raids on the island's southeastern coast.
- A fifteen-year plague begins in the Roman Empire.
- Shapur I orders an invasion of the Roman East with plans of finally taking the jewel of Syria, the city of Antioch.
- King Shapur I of Persia defeats a Roman at Barbalissos in Syria.
- Revolts in Khorasan start though are quickly put down by the Persian armies. Shapur I also invades Armenia and Georgia, annexing the latter and making it a special province. Artavazd VI is made the new Armenian king.
- The Period of the Thirty Tyrants starts in the Roman Empire. The legions who campaigned against the Goths on the Danube elect Marcus Aemilius Aemilianus as the new emperor. He advances his troops into the German territories, where he meets his opponent emperor Trebonianus Gallus and his son Volsianus.
- New Emperor Aemilianus is proclaimed "enemy of the State" by the Roman senate. Shortly afterwards, he is killed at Interamna Nahars in Umbria.
- Valerian becomes the new Emperor of the Roman Empire.
- The Roman Empire continues their negative interactions with the hostile neighboring tribes.
- Ma Jun, a Chinese engineer from Cao Wei, invents the south-pointing chariot, a path-finding directional compass vehicle that uses a differential gear instead of magnetics.
- The Goths invade Asia Minor and Dacia. The entire region of Dacia is lost for the Roman Empire, and shortly afterwards would become a Gothic kingdom.
- Berbers massacre Roman colonists in the Maghreb.
- The Alemanni and the Franks continue their invasion of the Roman Empire, now occupying Milan.
- The Sasanians invade Mesopotamia and Syria. Antioch is conquered, and numerous Roman cities are destroyed.
- Unity is finally restored in China with the victories of the Wei Kingdom of the north.