The Battle of Poetovio was fought in 388 between the forces of Western Roman Emperor Magnus Maximus and the Eastern Roman Emperor Theodosius I. Magnus Maximus's army was defeated and Maximus was later captured and executed at Aquileia.

Following the death of Emperor Valentinian I, his son Gratian, already co-emperor assumes the role of Emperor of the Western Roman Empire, while his other son, a child of four, Valentinian II is proclaimed Emperor. On 19 January 379, following the death of Emperor Valens at the Battle of Adrianople months before, Gratian appoints Theodosius as Emperor of the Eastern Roman Empire. Theodosius sets about containing the incursions by Goths into the empire.

In 383, Magnus Maximus is proclaimed Emperor in Roman Britain. Gratian, in the midst of campaigning against the Alemanni, hears of the revolt and marches his army towards Paris. Maximus quickly transports his army across the channel, landing at the mouth of the Rhine, and by the time Gratian arrives at Paris, Maximus is waiting. After a five day skirmish, Gratian's army deserts him. He flees, and is later murdered.

In 387, the eastern Roman empire experiences a general uprising following Theodosius's edict to raise taxes. Taking advantage of riots in major cities throughout the eastern Roman empire, Maximus invades Italy. Valentinian II and his court flee, first to Aquileia, then to Salonica to seek the protection of Theodosius.

Theodosius greeted the imperial court of Valentinian at Salonica. Faced with the choice of avoiding a civil war or supporting Valentinian, Theodosius chose Valentinian. Theodosius took his time gathering Alans, Goths, Huns, and Armenians into his army. He planned to attack Maximus in Italy, while marching with the main army into Pannonia. Along with his two-prong attack on Maximus's territory, Theodosius also stationed a sizable force in Egypt, due to demonstrations in favour of Maximus.

Maximus, in the meantime, had marched part of his army to Siscia commanded by Andragathius, blocking Theodosius's advance into Italy. When Maximus learned of Theodosius navy moving towards Italy, he ordered Andragathius to intercept, but he failed to stop Theodosius naval attack. Theodosius I arrived at Siscia with a large force consisting of Goths, Huns and Alans as cavalry.

A brief skirmish at Siscia occurred between the two armies. Theodosius's cavalry, consisting of Alans, Goths, and Huns, charged across the Sava river directly at Maximus's forces. Maximus's forces broke and ran, being massacred by Theodosian forces. Theodosius's army took very few casualties.

Theodosius continued his march and met Maximus's army under the command of his brother, Marcellinus, at Poetovio. The following day, the battle started with both armies' infantry fighting in squares. The cavalry of both armies appear to have negated each other, neither unable to gain an advantage. Maximus's infantry fought "with the desperation of gladiators", unwilling to yield ground. Finally Theodosius's infantry gained the upper hand and Maximus's troops either capitulated or fled. Maximus fled to Aquilia.

After his decisive victory Theodosius, following in swift pursuit, besieged Maximus in his refuge of Aquilia, a fortress west of the Julian Alps. The garrison soon surrendered, delivering Maximus to Theodosius. The death of Maximus (August 28th), and of his son Victor (captured and executed by Arbogastes), ended the conflict. Now in command of the entire Roman world, Theodosius planned to court the Roman nobility during his stay in Italy (388–391).

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46°25′07″N 15°52′17″E / 46.4186°N 15.8714°E / 46.4186; 15.8714