The bishops of Trier were already virtually independent territorial magnates in Merovingian times.
In 772 Charlemagne granted Bishop Wiomad complete immunity from the jurisdiction of the ruling count for all the churches and monasteries, as well as villages and castles that belonged to the Church of St. Peter at Trier.
In his will he also elevated the diocese to the Archdiocese of Trier, with suffragans on both sides of the Rhine.
This arrangement lasted over a thousand years.

In Early Modern times, the archdiocese of Trier still encompassed territory along the Moselle River between Trier, near the French border, and Koblenz on the Rhine.
 The Archbishop of Trier, as holder of an imperial office was traditionally an Imperial Elector of the German king.
The purely honorary office of Archchancellor of Gaul arose in the 13th century.
In this context that was taken to mean the Kingdom of Arles, or Burgundy, technically from 1242 and permanently from 1263, and nominally until 1803.
Arles along with Germany and Italy was one of the three component kingdoms of the Empire.

The last elector removed to Koblenz in 1786.
From 1795, the territories of the Archbishopric on the left bank of the Rhine — which is to say almost all of them — were under French occupation, and were annexed in 1801 and a separate bishopric established (later assuming control of the whole diocese in 1803).
 In 1803, what was left of the Archbishopric was secularized and annexed by the Princes of Nassau.
The bishops of Trier were already virtually independent territorial magnates in Merovingian times. In 772 Charlemagne granted Bishop Wiomad complete immunity from the jurisdiction of the ruling count for all the churches and monasteries, as well as villages and castles that belonged to the Church of St. Peter at Trier. In his will he also elevated the diocese to the Archdiocese of Trier, with suffragans on both sides of the Rhine. This arrangement lasted over a thousand years. 
In Early Modern times, the archdiocese of Trier still encompassed territory along the Moselle River between Trier, near the French border, and Koblenz on the Rhine.  The Archbishop of Trier, as holder of an imperial office was traditionally an Imperial Elector of the German king. The purely honorary office of Archchancellor of Gaul arose in the 13th century. In this context that was taken to mean the Kingdom of Arles, or Burgundy, technically from 1242 and permanently from 1263, and nominally until 1803. Arles along with Germany and Italy was one of the three component kingdoms of the Empire.
The last elector removed to Koblenz in 1786. From 1795, the territories of the Archbishopric on the left bank of the Rhine — which is to say almost all of them — were under French occupation, and were annexed in 1801 and a separate bishopric established (later assuming control of the whole diocese in 1803).  In 1803, what was left of the Archbishopric was secularized and annexed by the Princes of Nassau.
Roman Catholic Diocese of Trier