Client states in history
Persia, Greece, and Rome
Ancient states such as Persia and Greek city-states would create client states by making the leaders of that state subservient. Classical Athens, for example, forced weaker states into the Delian League and in some cases imposed democratic government on them. Later, Philip II of Macedon similarly imposed the League of Corinth. One of the most prolific users of client states was Republican Rome which, instead of conquering and then absorbing into an empire, chose to make client states out of those it defeated (e.g. Demetrius of Pharos), a policy which was continued up until the 1st century BCE when it became the Roman Empire. Sometimes the client was not a former enemy but a pretender whom Rome helped, Herod the Great being a well-known example. The use of client states continued through the Middle Ages as the feudal system began to take hold.
Under the Mongols and the Yuan dynasty
In the 13th century, Goryeo dynasty of Korea was overrun by the Mongols who founded the powerful Mongol Empire. After the peace treaty in 1260 and the Sambyeolcho Rebellion in 1270, Goryeo became a semi-autonomous client state of the Yuan dynasty for about 80 years.