AD 283: Exploiting the Persian civil war, Carus leaves Carinus in charge of much of the Roman Empire and, accompanied by his younger son Numerian, invades the Sassanid Empire. They sack Seleucia and Ctesiphon, the capital of the Persian kingdom, and they press on beyond the Tigris. For his victories, Carus receives the title of Persicus Maximus.
AD 287: Around this time, the future emperor Constantius defeats and captures a Germanic king, the latter having prepared an ambush against the Romans.
AD 289: In this or the following year, Emperor Diocletian campaigns with success against the Sarmatians. The future emperor Galerius may have distinguished himself during this campaign.
AD 290: Following his victory over Emperor Maximian's fleet, the usurper Carausius invades the European mainland and re-establishes his military and administrative presence in northern Gaul.
AD 293: King Bahram II of the Persian Empire dies after a 17-year reign; his son Bahram III ascends to the throne. After four months, Bahram III's great-uncle Narseh, the king of Persarmenia, marches on the Persian capital Ctesiphon with the support of a faction of the nobility and the eastern Satraps. Bahram is overthrown and Narseh is declared the new King of Kings.
AD 297: Diocletian watches over the Syrian provinces while Caesar Galerius makes preparations for a campaign against the Persian king Narseh. He recruits veterans from Illyria and Moesia, recruits new soldiers, and strengthens his army with Gothic mercenaries and the Armenian units of Tiridates III.
AD 321: March 7 - Constantine I signs legislation directing urban residents to refrain from work, and businesses to be closed, on the "venerable day of the Sun". An exception is made for agriculture.