Historically ruled by various Celtic tribes, the territory of the Saar was conquered by the Roman Empire in the first century BCE, and was subsequently occupied by and annexed into the Carolingian Empire. Divided amongst many states of the Holy Roman Empire of the German Nation, the territory was invaded by French revolutionaries and annexed to the French Republic in 1792. Restored to various German states by the Congress of Vienna in 1815, it was part of Prussia within the German Empire from 1871 until 1918.