The siege of Segusio () or siege of Susa was the first clash of the

civil war A civil war is a war between organized groups within the same Sovereign state, state (or country). The aim of one side may be to take control of the country or a region, to achieve independence for a region, or to change government policies.J ...

between the Roman emperors

Constantine the Great Constantine I (27 February 27222 May 337), also known as Constantine the Great, was a Roman emperor from AD 306 to 337 and the first Roman emperor to convert to Christianity. He played a Constantine the Great and Christianity, pivotal ro ...

() and

Maxentius Marcus Aurelius Valerius Maxentius ( 283 – 28 October 312) was a Roman emperor from 306 until his death in 312. Despite ruling in Italy and North Africa, and having the recognition of the Senate in Rome, he was not recognized as a legitimate ...

() in the spring of

312 __NOTOC__ Year 312 (Roman numerals, CCCXII) was a leap year starting on Tuesday of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Constantinus and Licinianus (or, less frequently, year 1065 ''Ab urbe co ...

. In that year,

had declared war on

Constantine Constantine most often refers to: * Constantine the Great, Roman emperor from 306 to 337, also known as Constantine I * Constantine, Algeria, a city in Algeria Constantine may also refer to: People * Constantine (name), a masculine g ...

, claiming to intend to avenge the death of his father

Maximian Maximian (; ), nicknamed Herculius, was Roman emperor from 286 to 305. He was ''Caesar (title), Caesar'' from 285 to 286, then ''Augustus (title), Augustus'' from 286 to 305. He shared the latter title with his co-emperor and superior, Diocleti ...

(), who had committed suicide after being defeated by him.

would respond with an invasion of northern Italy. The siege of Segusio was the first of many battles that took place in the Italian peninsula, in all of them

was victorious. With the capture of the city after subjecting it to a siege, the victorious emperor was able to continue his march through the interior of the peninsula until he reached

Augusta Taurinorum Turin ( , ; ; , then ) is a city and an important business and cultural centre in northern Italy. It is the capital city of Piedmont and of the Metropolitan City of Turin, and was the first Italian capital from 1861 to 1865. The city is main ...

, where he fought the

Battle of Turin The siege of Turin took place from June to September 1706, during the War of the Spanish Succession. A French army led by Louis de la Feuillade besieged the Savoyard capital of Turin, whose relief by Prince Eugene of Savoy has been called th ...

against the forces of

Background

At the end of the

3rd century The 3rd century was the period from AD 201 (represented by the Roman numerals CCI) to AD 300 (CCC) in accordance with the Julian calendar. In this century, the Roman Empire saw a crisis, starting with the assassination of the Roman Emperor ...

, the

Roman Empire The Roman Empire ruled the Mediterranean and much of Europe, Western Asia and North Africa. The Roman people, Romans conquered most of this during the Roman Republic, Republic, and it was ruled by emperors following Octavian's assumption of ...

was divided into East and West, and each part was ruled by an

Augustus Gaius Julius Caesar Augustus (born Gaius Octavius; 23 September 63 BC – 19 August AD 14), also known as Octavian (), was the founder of the Roman Empire, who reigned as the first Roman emperor from 27 BC until his death in A ...

, or major emperor, and a

Caesar Gaius Julius Caesar (12 or 13 July 100 BC – 15 March 44 BC) was a Roman general and statesman. A member of the First Triumvirate, Caesar led the Roman armies in the Gallic Wars before defeating his political rival Pompey in a civil war. He ...

, or minor emperor. After the

of the East,

Galerius Galerius Valerius Maximianus (; Greek: Γαλέριος; 258 – May 311) was Roman emperor from 305 to 311. He participated in the system of government later known as the Tetrarchy, first acting as '' caesar'' under Emperor Diocletian. In th ...

, bestowed the dignity of

(), son of Emperor Maximian Herculius (), he married

' daughter,

Valeria Maximilla Valeria Maximilla () was a Roman Empress and wife of Emperor Maxentius. Life Maximilla was the daughter of Emperor Galerius and his first wife, whose name is unknown. Around 293 (the exact date is unknown), she married Maxentius, son of Empero ...

. On October 28,

306 __NOTOC__ Year 306 ( CCCVI) was a common year starting on Tuesday of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantius and Valerius (or, less frequently, year 1059 ''Ab urbe condita''). The denominatio ...

proclaimed himself ''

princeps ''Princeps'' (plural: ''Principes'') is a Latin word meaning "first in time or order; the first, foremost, chief, the most eminent, distinguished, or noble; the first person". As a title, ''Princeps'' originated in the Roman Republic wherein the ...

''. The following year, the young emperor assumed the title of

, forcing his father

to ratify it to give his rule the appearance of legitimacy. As soon as

heard of his son-in-law's actions, he sent Emperor Flavius Severus to put down the rebellion.

, with the help of a ruse by his father

, managed to take Severus prisoner. When Flavius Severus died,

invaded Italy to try to avenge his death, but realizing that

would be trying to win over his army, he chose not to trust his own troops and withdrew. Between 308 and 310,

tried to assume all power, to the detriment of his son

, without achieving his goal. He then tried to win

to his cause, but this plan also failed, then he tried to win

Diocletian Diocletian ( ; ; ; 242/245 – 311/312), nicknamed Jovius, was Roman emperor from 284 until his abdication in 305. He was born Diocles to a family of low status in the Roman province of Dalmatia (Roman province), Dalmatia. As with other Illyri ...

to his side at

Carnuntum Carnuntum ( according to Ptolemy) was a Roman legionary fortress () and headquarters of the Roman navy, Pannonian fleet from 50 AD. After the 1st century, it was capital of the Pannonia Superior province. It also became a large city of app ...

. When his intrigues were frustrated, Maximian Herculius returned to the side of his son-in-law

Gaul Gaul () was a region of Western Europe first clearly described by the Roman people, Romans, encompassing present-day France, Belgium, Luxembourg, and parts of Switzerland, the Netherlands, Germany, and Northern Italy. It covered an area of . Ac ...

, where he died in 310 after being implicated in a plot against him. The following year,

, demanding revenge for the death of his father, declared war on

, who, in response, invaded northern

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

, mobilizing 40,000 soldiers, according to some sources. According to the chronicles of the historian

Zosimus Zosimus, Zosimos, Zosima or Zosimas may refer to: People * * Rufus and Zosimus (died 107), Christian saints * Zosimus (martyr) (died 110), Christian martyr who was executed in Umbria, Italy * Zosimos of Panopolis, also known as ''Zosimus Alch ...

, the Constantinian forces consisted of 90,000 legionaries and 8,000 horsemen.

Battle and consequences

The first town

's army encountered on its way to

Rome Rome (Italian language, Italian and , ) is the capital city and most populated (municipality) of Italy. It is also the administrative centre of the Lazio Regions of Italy, region and of the Metropolitan City of Rome. A special named with 2, ...

was

Segusio Susa (, , ) is a town and ''comune'' in the Metropolitan City of Turin, Piedmont (Italy), Piedmont, Italy. In the middle of Susa Valley, it is situated on at the confluence of the Cenischia with the Dora Riparia, a tributary of the Po River, at t ...

(present-day

Susa Susa ( ) was an ancient city in the lower Zagros Mountains about east of the Tigris, between the Karkheh River, Karkheh and Dez River, Dez Rivers in Iran. One of the most important cities of the Ancient Near East, Susa served as the capital o ...

), a city heavily walled during the refortification program of northern Italian cities the previous year. Rather than bow to the invading army, the

citizens Citizenship is a membership and allegiance to a sovereign state. Though citizenship is often conflated with nationality in today's English-speaking world, international law does not usually use the term ''citizenship'' to refer to nationality; ...

closed their gates to

. The emperor then ordered his men to set fire to its great gates and scale its walls. The city was quickly taken, but

prevented his troops from sacking the city, preferring to continue his advance into the interior of

. With the conquest of

, the Constantinian army marched in the direction of

(present-day

Turin Turin ( , ; ; , then ) is a city and an important business and cultural centre in northern Italy. It is the capital city of Piedmont and of the Metropolitan City of Turin, and was the first Italian capital from 1861 to 1865. The city is main ...

), where an army composed mainly of

heavy cavalry Heavy cavalry was a class of cavalry intended to deliver a battlefield charge and also to act as a Military reserve, tactical reserve; they are also often termed ''shock cavalry''. Although their equipment differed greatly depending on the re ...

was encamped. In the vicinity of this city the

was fought, where

would obtain a new victory, which opened the way to the other important cities of the

Padan Plain The Po Valley, Po Plain, Plain of the Po, or Padan Plain (, , or ) is a major geographical feature of northern Italy. It extends approximately in an east-west direction, with an area of including its Venetian Plain, Venetic extension not actu ...

. Subsequently, a series of triumphs would follow, as in

Brescia Brescia (, ; ; or ; ) is a city and (municipality) in the region of Lombardy, in Italy. It is situated at the foot of the Alps, a few kilometers from the lakes Lake Garda, Garda and Lake Iseo, Iseo. With a population of 199,949, it is the se ...

and

Verona Verona ( ; ; or ) is a city on the Adige, River Adige in Veneto, Italy, with 255,131 inhabitants. It is one of the seven provincial capitals of the region, and is the largest city Comune, municipality in the region and in Northeast Italy, nor ...

. The surrender of the latter city led to the unconditional submission of

Aquileia Aquileia is an ancient Roman city in Italy, at the head of the Adriatic at the edge of the lagoons, about from the sea, on the river Natiso (modern Natisone), the course of which has changed somewhat since Roman times. Today, the city is small ( ...

Mutina Modena (, ; ; ; ; ) is a city and ''comune'' (municipality) on the south side of the Po Valley, in the Province of Modena, in the Emilia-Romagna region of northern Italy. It has 184,739 inhabitants as of 2025. A town, and seat of an archbi ...

(now

Modena Modena (, ; ; ; ; ) is a city and ''comune'' (municipality) on the south side of the Po Valley, in the Province of Modena, in the Emilia-Romagna region of northern Italy. It has 184,739 inhabitants as of 2025. A town, and seat of an archbis ...

) and

Ravenna Ravenna ( ; , also ; ) is the capital city of the Province of Ravenna, in the Emilia-Romagna region of Northern Italy. It was the capital city of the Western Roman Empire during the 5th century until its Fall of Rome, collapse in 476, after which ...

. Thus the road to Rome was open to

. The

Battle of the Milvian Bridge The Battle of the Milvian Bridge took place between the Roman Emperors Constantine the Great, Constantine I and Maxentius on 28 October AD 312. It takes its name from the Milvian Bridge, an important route over the Tiber. Constantine won the batt ...

was the last confrontation of the war between the two emperors.

died in battle and his head was paraded through Rome. Constantine thus became the only emperor of the western part of the

References

Bibliography

* Anonymous (3rd-4th centuries). ''

Panegyrici Latini ' or ''Twelve Latin Panegyrics'' is the conventional title of a collection of twelve ancient Roman and late antique prose panegyric orations written in Latin. The authors of most of the speeches in the collection are anonymous, but appear to hav ...

.'' * * * * * * * * * * * {{Cite book , last=Ridley , first=Ronald T. , url=https://books.google.com/books?id=MvA0DwAAQBAJ&q=Zosimus+-+R.+T.+Ridley%3A+Zosimus%2C+New+History , title=Zosimus, New History. A Translation with Commentary , publisher=

Brill Brill may refer to: Places * Brielle (sometimes "Den Briel"), a town in the western Netherlands * Brill, Buckinghamshire, a village in England * Brill, Cornwall, a small village to the west of Constantine, Cornwall, UK * Brill, Wisconsin, an un ...

, year=2017 , isbn=9780959362602 312 4th century in Italy 310s conflicts Battles involving the Roman Empire Battles of Constantine the Great 310s in the Roman Empire Sieges involving ancient Rome Military history of Piedmont Province of Turin