Emperor Diocletian campaigns with success against Arabic enemies.
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.
Asia
May 16 – Emperor Wu of Jin, founder of the Western Jin Dynasty, dies after a 25-year reign. He reunifies north and south, but gives away many dukedoms to his kinsmen. Crown Prince Sima Zhong succeeds his father, and has to deal with conflicts among the aristocratic families in China.
An uneasy peace is established between the emperors Diocletian and Maximian on the one hand, and the rival emperor Carausius on the other.
Perhaps in cooperation with the forces of Maximian, Carausius campaigns successfully against Germanic raids in Gaul and Britain. Also during his reign, Carausius begins building the forts of the Saxon Shore.
Northern Europe
The Alemanni, having been expelled from part of their territory by the Burgundians, seek to regain their lost lands. These peoples had unsuccessfully invaded Gaul in tandem in 285/6, and the Alemanni had likely been weakened by the Roman counter-invasions of 287 and 288.
King Bahram II fights against a coalition consisting of his brother Hormizd of Sakastan, the Kushano-Sasanians, and the Gilans.
China
War of the Eight Princes: After the death of Emperor Sima Yan (Jin Wudi), a civil war breaks out among the princes and dukes of the Jin Dynasty. The struggle devastates and depopulates the provinces of northern China.
March 1 – Emperors Diocletian and Maximian appoint Constantius I and Galerius as Caesars. This is considered the beginning of the Tetrarchy, known as the Quattuor Principes Mundi ("Four Rulers of the World"). (Some sources and scholars date Galerius' elevation to May 21.)
Constantius retakes some of the Gallic territories from the usurper Carausius. He conquers the crucial port of Bononia (modern Boulogne).
Towards the end of the year, Carausius is murdered by his finance minister Allectus, who proclaims himself the new emperor of Britain.
Galerius begins a series of two campaigns in Upper Egypt against the rebel cities of Coptos and Boresis as well as the Blemmyes and Meroitic Nubians.
Over the course of his reign, but especially from the time of the Tetrarchy's creation, Diocletian divides the large provinces of the early empire into smaller administrative units, and he groups these new smaller provinces into dioceses. He also accelerates the third-century trend whereby the administration and military of the provinces are increasingly divided between governors and generals (duces) respectively, whereas formerly governors had also been in charge of the legions. This expansion of imperial personnel increases Diocletian's control over the empire and weakens the power of individual officials and officers. Moreover, Diocletian expands the retinues of the individual emperors to have more ministers and secretaries, thus establishing what will become known as the late Roman Consistorium.
Persia
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.
Caesar Galerius completes a series of two campaigns in Upper Egypt, against the rebel cities of Coptos and Boresis, as well as the Blemmyes and Meroitic Nubians.
In this or the previous year, Caesar Constantius I assembles two invasion fleets with the intent of overthrowing the usurper Allectus, who is based in Britain. The first is under the command of Asclepiodotus, Maximian's long-serving Praetorian Prefect. Asclepiodotus sails from the mouth of the Seine, and lands near the Isle of Wight, where his forces defeat Allectus in Hampshire. Allectus is killed in the fighting. Constantius leaves Boulogne with his fleet, and occupies London, where he slaughters some of Allectus' Frankish mercenaries. With this victory, the Romano-British regime first established by Carausius is overthrown, and Britain is re-incorporated into the rest of the empire.
Having supervised the Rhine frontier during Constantius' invasion of Britain, Maximian then marches into Spain, where he fights Frankish pirates. He then crosses into North Africa to contend with the rebellion of the Quinquegentiani.
The Persian king Narseh invades Roman-held Upper Mesopotamia and Arsacid western Armenia, the latter territory being under the leadership of the pro-Roman king Tiridates III. With only a small army, Caesar Galerius fights three holding actions against Narseh's army in Mesopotamia. Somewhere in the open plains between Carrhae and Callinicum, Galerius' army suffers a defeat against the Persian army, which is both more numerous and contains superior numbers of high-quality cavalry. Nevertheless, Galerius succeeds in blunting the Persian offensive.
Emperor Diocletian introduces a new tax system and other economic reforms.[6]
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.
August: Domitius Domitianus launches a usurpation against Diocletian in Egypt. He is perhaps aided by popular discontent with Diocletian's taxation reform.
Autumn: Diocletian besieges the rebels in Alexandria.
December: Domitianus dies, but his correctorAurelius Achilleus takes over as the leader of the rebellion.
Battle of Satala: Galerius launches a surprise attack against Narseh's camp in western Armenia. The Romans sack the camp and capture Narseh's wives, sisters and daughters, including his Queen of Queens Arsane. Narseh is wounded and escapes to his empire.
Diocletian then travels into Upper Egypt and possibly campaigns on the Nubian frontier. In either this year or in 300/301, he makes agreements with the Meroitic Nubians and the Blemmyes. He agrees to pay subsidies to both peoples, and he cedes the Dodecashoenos to the Nubians on the understanding that the Nubians will defend the region against the Blemmyes.
Peace of Nisibis: Emperor Diocletian signs a treaty with the Persian king Narseh that will last for 40 years. The Persians accept Roman dominion over Armenia, the Caucasus, and Upper Mesopotamia. The pro-Roman ruler Tiridates III receives all of Armenia as far as the border with Atropatene. Mirian III of the Kingdom of Iberia is made a Roman client, and at some point in time, as a result of the treaty, Caucasian Albania will follow suit. Rome also gains five satrapies beyond the Tigris, which are perhaps given to Tiridates to administer.
In this or the following year, Galerius campaigns with success against Sarmatians and the Marcomanni, attacking through a swamp to defeat a Sarmatian army.
Having first crossed into Africa in 296, Emperor Maximian concludes his campaigns against the Quinquegentiani and other Berbers. His campaigns had ranged as far as Mauretania in the west and Tripolitania in the east. Julianus, a rebel leader in Africa, throws himself into a fire after the Romans breach the walls of his stronghold.
Returning to Rome in triumph, Maximian commissions the Baths of Diocletian in honour of his 'brother' Diocletian.
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