A client state is a state that is economically, politically or militarily subordinate to another more powerful state in international affairs. Types of client states include: satellite state, associated state, puppet state, neo-colony, protectorate, vassal state and tributary state.
Client states in antiquity 
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. One of the most prolific users of client states was Republican Rome (e.g., Demetrius of Pharos) which, instead of conquering and then absorbing into an empire, chose to make client states out of those it defeated, a policy which was continued up until the 1st century BC when it became the Roman Empire. The use of client states continued through the Middle Ages as the feudal system began to take hold.
Under the Mongol Empire 
19th and 20th centuries 
Client states of France 
During the French Revolution and Napoleonic eras, France conquered most of the western Europe and established several client states. At first, during the French revolutionary wars these states were erected as republics (the so-called "Républiques soeurs", or "sisters republics"). They were established in Italy (Cisalpine Republic in Northern Italy, Parthenopean Republic in Southern Italy), Switzerland, Belgium, and the Netherlands as a republic and a monarchy.
During the First French Empire, while Napoleon and the French army conquered Europe, such states changed, and several new states were formed. The Italian republics were transformed into the Kingdom of Italy, under Napoleon's direct rule, in the north and the Kingdom of Naples in the south, under Joseph Bonaparte's rule and later under Marshal of the Empire Joachim Murat's rule.
The west bank of the River Rhine was annexed and was a part of the French Empire. Numerous German states, comprising the Confederation of the Rhine, were client states of the French Empire, including the Kingdom of Westphalia, which was controlled by Napoleon's brother.
The British Empire 
In the British empire the Indian Princely States were technically independent (and were technically given their separate independence in 1947, although the Nizam of Hyderabad did not retain his independence from India). Egyptian Independence in 1922 technically ended a British protectorate in Egypt. Sudan and Egypt continued to be governed as Anglo-Egyptian Sudan until Sudanese independence in 1956; Britain also had an interest in Egypt until the Suez Crisis was over. Iraq was made a kingdom in 1932. In each case the economic and military reality did not amount to full independence, but a status where the local rulers were British clients. Similarly in Africa (e.g. Northern Nigeria under Lord Lugard), and Malaya with the Federated Malay States and Unfederated Malay States; the policy of indirect rule.
United States 
After 1945 the term was often applied to nations ruled by dictatorships backed openly by either the United States or the Soviet Union. During the Cold War, many Latin American nations such as Guatemala, El Salvador, Nicaragua until 1979, Cuba until 1959, and Chile under the regime of General Augusto Pinochet were seen as U.S. client states, as the U.S. government had significant influence over the policies of those dictatorships. The term also applied to other authoritarian regimes with close ties to the United States during the Cold War, more appropriately referred to as U.S. proxy states, such as South Vietnam, Iran until 1979, Cambodia under the regime of Lon Nol, the Philippines, and Saudi Arabia. A good case study of client state building is the U.S. - Iran relations under the Shah.
The term might also arguably be used for those states extremely economically dependent on a more powerful nation. The three Pacific ocean countries associated with the United States under the Compact of Free Association (the Federated States of Micronesia, the Marshall Islands and Palau) may fall somewhat in this category.
The United States also supports small, but strategic, countries in regions where their sovereignty is threatened. Israel, the Republic of China (Taiwan), South Korea and Bahrain are good examples. Israel's strategic location in the Eastern Mediterranean and near the Suez Canal, Taiwan's strategic location as a naval base of operations in the northwestern Pacific Ocean and putting an American presence in the South China Sea, South Korea's sovereignty was threatened by North Korea and this puts American presence in the Sea of Japan and Bahrain's location in the vital Persian Gulf and near the Strait of Hormuz, make American interests in good relations with these small nations very prominent.
Soviet Union 
Soviet proxy or "client" states included much of the Warsaw Pact nations whose policies were heavily influenced by Soviet military power and economic aid. Other third world nations with Marxist-Leninist governments were routinely criticized as being Soviet proxies as well, among them Cuba following the Cuban Revolution, the People's Republic of Angola, the People's Republic of Mozambique, the Democratic Republic of Afghanistan, and the Democratic Republic of Vietnam (North Vietnam). Within the Soviet Union itself, the Ukrainian SSR and the Byelorussian SSR had seats at the United Nations, but were actually proper Soviet territory.
- Michael Graham Fry, Erik Goldstein, Richard Langhorne. Guide to International Relations and Diplomacy. London, England, UK; New York, New York, USA: Continuum International Publishing, 2002. Pp. 9.
- Herod's Judaea
- Collected studies: Alexander and his successors in Macedonia, by Nicholas Geoffrey Lemprière Hammond,1994,page 257,"to Demetrius of Pharos, whom she set up as a client king
- Gasiorowski, Mark US Foreign Policy and the Shah, Cornell University Press, 1991