Augusta Treverorum in the context of "Crispus"

Trier (/trɪər/ TREER, German: [tʁiːɐ̯] ; Luxembourgish: Tréier [ˈtʀɜɪ̯ɐ] ), formerly and traditionally known in English as Trèves (/trɛv/ TREV, French: [tʁɛv] ) and Triers (see also names in other languages), is a city on the banks of the Moselle in Germany. It lies in a valley between low vine-covered hills of red sandstone in the west of the state of Rhineland-Palatinate, near the border with Luxembourg and within the important Moselle wine region.

Founded by the Romans in the late 1st century BC as Augusta Treverorum ("The City of Augustus among the Treveri"), Trier is considered Germany's oldest city. It is also the oldest seat of a bishop north of the Alps. Trier was one of the four capitals of the Roman Empire during the Tetrarchy period in the late 3rd and early 4th centuries. In the Middle Ages, the archbishop-elector of Trier was an important prince of the Church who controlled land from the French border to the Rhine. The archbishop-elector of Trier also had great significance as one of the seven electors of the Holy Roman Empire. Because of its significance during the Roman and Holy Roman empires, several monuments and cathedrals within Trier are listed as a UNESCO World Heritage Site.

The Treveri (Gaulish: *Treweroi) were a Germanic or Celtic tribe of the Belgae group who inhabited the lower valley of the Moselle in modern day Germany from around 150 BCE, if not earlier, until their displacement by the Franks. Their domain lay within the southern fringes of the Silva Arduenna (Ardennes Forest), a part of the vast Silva Carbonaria, in what are now Luxembourg, southeastern Belgium and western Germany; its centre was the city of Augusta Treverorum (Trier), to which the Treveri give their name. Celtic in language, according to Tacitus they claimed Germanic descent. They contained both Gallic and Germanic influences.

Although early adopters of Roman material culture, the Treveri had a chequered relationship with Roman power. Their leader Indutiomarus led them in revolt against Julius Caesar during the Gallic Wars; much later, they played a key role in the Gaulish revolt during the Year of the Four Emperors. On the other hand, the Treveri supplied the Roman army with some of its most famous cavalry, and the city of Augusta Treverorum was home for a time to the family of Germanicus, including the future emperor Gaius (Caligula). During the Crisis of the Third Century, the territory of the Treveri was overrun by Germanic Alamanni and Franks and later formed part of the Gallic Empire.

Mogontiacum (also Moguntiacum) is the Latin name of today's city of Mainz, which it bore during its almost 500 years as part of the Roman Empire. Mogontiacum had its origins in the legionary camp built by Drusus in 13/12 BCE, which was strategically located on a hill [de] above the Rhine and opposite the mouth of the Main on the Roman Rhine valley road [de].

The civilian settlements (vici) in the vicinity of the camp, which spread down the Rhine, quickly grew together to form a larger, urbanised settlement. However, unlike Colonia Claudia Ara Agrippinensium (Cologne) or Augusta Treverorum (Trier), Mogontiacum was primarily a military centre until the second half of the 4th century and was apparently not a colonia either. As a result, the city never had the urban character of the other large Roman cities in Germany. Nevertheless, several monumental buildings were also erected here, as Mogontiacum was the provincial capital of the Roman province of Germania Superior with the seat of the governor as of the year 90 at the latest. After the middle of the 3rd century, when the Decumatian Fields were cleared, Mogontiacum once again became a border town and was devastated several times over the next 150 years by members of various Germanic peoples. After the end of the Roman period, but at the latest around 470, Mogontiacum belonged to the Frankish Kingdom after a brief transitional phase.

The Diocese of Trier (Latin: Dioecesis Trevirensis), in English historically also known as Treves (/triːvz/) from French Trèves, is a Latin Catholic ecclesiastical territory or diocese of the Catholic Church in Germany. When it was the archbishopric and Electorate of Trier, it was one of the most important states of the Holy Roman Empire, both as an ecclesiastical principality and as a diocese of the church. Unlike the other Rhenish dioceses—including Mainz and Cologne–Trier was the former Roman provincial capital of Augusta Treverorum. Given its status, Trier has continuously been an episcopal see since Roman times and is one of the oldest dioceses in all of Germany. The diocese was elevated to an archdiocese in the time of Charlemagne and was the metropolitan for the dioceses of Metz, Toul, and Verdun. After the victory of Napoleon Bonaparte of France, the archdiocese was lowered to a diocese and is now a suffragan in the ecclesiastical province of the Archdiocese of Cologne. The diocesan cathedral is the Cathedral of Saint Peter. The Cathedral Chapter retains the right to elect the bishop, rather than selection by papal appointment.