Gallic Empire

State · 261–282 CE · at its largest around 261 CE

Breakaway Roman state in Gaul, Britain and for a time Hispania, founded by Postumus during the 3rd-century crisis and reconquered by Aurelian.

On its lands while it lasted

• Trier (16 BCE – 1618 CE): Capital of Electorate of Trier (1333–1618).
• Cologne (c. 261 CE): Capital of Gallic Empire (261–274).

More on Wikipedia.