Assassination attempt · March 235 · Moguntiacum (Mainz), Germania Theater

On the German frontier, the young emperor Severus Alexander — the last of the Severan dynasty — was murdered by his own mutinous soldiers, who resented his attempts to buy off the barbarians rather than fight them. They proclaimed instead a giant Thracian officer risen from the ranks, Maximinus Thrax, the first of the 'soldier-emperors.'

Battle · 238 · Rome, Africa, and the frontier Theater

In a single chaotic year, six men claimed the imperial title. A revolt in Africa raised the elderly Gordian I and his son, who quickly fell; the Senate then named Pupienus and Balbinus to resist Maximinus Thrax, who marched on Italy but was murdered by his own starving troops besieging Aquileia. Pupienus and Balbinus were then killed by the Praetorian Guard, leaving the boy Gordian III on the throne.

Battle · 249 – 262 · Across the Roman Empire Theater

A devastating pandemic — possibly a viral haemorrhagic fever — swept the empire for more than a decade, described in horrifying detail by the Christian bishop Cyprian of Carthage. At its height it was said to kill 5,000 people a day in Rome alone, emptying cities, ravaging the armies, and crippling the economy at the very moment the empire faced invasion on every front.

Battle · June 251 · Moesia (modern Bulgaria) Theater

The emperor Decius and his son led a Roman army against a great Gothic invasion of the Balkans. Lured into marshy ground at Abritus, the Roman army was ambushed and annihilated; Decius's son was killed by an arrow, and the emperor himself perished in the bog — the first Roman emperor ever to be killed in battle against a foreign enemy.

Battle · 260 · Mesopotamia (Edessa) Theater

Marching east to halt the resurgent Sasanian Persians, the emperor Valerian was defeated and — after his army was broken by plague and the enemy near Edessa — captured alive by King Shapur I during a parley. He spent the rest of his life in humiliating captivity, used, by legend, as Shapur's living footstool, and after death his flayed and stuffed skin was hung as a trophy in a Persian temple.

Battle · 260 – 274 · Gaul, Britain, and Hispania Theater

As the central government reeled, the western provinces broke away. The general Postumus proclaimed a separate 'Gallic Empire' comprising Gaul, Britain, and Spain, with its own emperor, senate, and capital — not to overthrow Rome, but to defend the west against the Germanic invaders that the central emperors could no longer repel. It lasted fourteen years under a succession of rulers.

Battle · 267 – 272 · Syria, Egypt, and Asia Minor Theater

In the east, where the capture of Valerian had left a vacuum, the desert city of Palmyra rose to power. Its brilliant queen Zenobia, ruling for her young son, seized Syria, then conquered Egypt — Rome's breadbasket — and much of Asia Minor, carving out a Palmyrene Empire that rivalled Rome itself. Cultured, ambitious, and claiming descent from Cleopatra, she made her court a centre of learning.

Battle · 268 or 269 · Moesia (Niš, modern Serbia) Theater

A large Gothic force crossed the Danube and raided the Balkans and Aegean. Roman forces defeated it near Naissus, but the confused later sources do not permit a fully secure reconstruction: historians dispute whether the engagement belongs in 268 or 269 and whether Gallienus or Claudius II held overall command. Later accounts report tens of thousands of Gothic dead, a figure that should be treated as literary testimony rather than a verified count.

Battle · 271 – 275 · Rome Theater

For the first time in centuries, the city of Rome itself felt unsafe. After Germanic raiders penetrated deep into Italy, Aurelian ordered the construction of a massive new circuit of walls around the capital — nineteen kilometres of brick-faced concrete, the largest fortification project in the city's history — a stark admission that even Rome was no longer beyond the reach of the enemy.

Battle · 272 · Syria (Emesa, modern Homs) Theater

The energetic soldier-emperor Aurelian marched east to reclaim the Roman provinces from Palmyra. After beating the Palmyrene heavy cavalry at Immae by feigning retreat to exhaust their armoured horsemen, he defeated Zenobia's main army at Emesa and drove her back to Palmyra, which he then besieged and took. Zenobia was captured trying to flee to Persia.

Battle · 274 · Gaul (Châlons) Theater

Having recovered the East, Aurelian turned west against the Gallic Empire. At Châlons he defeated the army of Tetricus — who, weary of ruling a doomed state, may have secretly arranged his own defeat — and reunited Gaul, Britain, and Spain with the empire. In a single reign, Aurelian had reassembled the shattered Roman world.

Battle · November 284 · Nicomedia (Asia Minor) Theater

After the death of Numerian, the army proclaimed the Illyrian officer Diocletian emperor near Nicomedia on 20 November 284. Carinus still ruled in the West; their armies did not meet until the Battle of the Margus in 285. Diocletian subsequently reconstructed imperial government, taxation, coinage, and military organization and later shared authority through the Tetrarchy.