Rump state

A rump state is the remnant of a once-larger government, left with limited powers or authority after a disaster, invasion or military occupation.

For example, the Seleucid Empire, which once controlled most of the Middle-east, was reduced drastically in size by the rise of the Parthian Empire in its eastern provinces due to a local rebellion. What was left of the once large empire was a rump state composed of little more than Antioch, Damascus, and an area roughly equal in size to modern Syria.