Sphere of influence, or sometimes zone of influence or sphere of interest, is a diplomatic term denoting an area in which a foreign power or powers exerts significant military, cultural, or economic influence.
This concept became recognized in international law during the scramble for Africa" in the 1880s, when the great powers carved up the continent for commercial exploitation. Later it was applied to the Far East, notably in China.
The Open Door policy, while appearing on the surface to be anticolonial, tended to support the existence of spheres of influence.
Sometimes, a sphere of influence can be incorporated into a treaty, such as the control exercised by the Soviet Union in Eastern Europe between the end of World War II and the fall of communism in 1989. Eastern European countries belonged to the formal Warsaw Pact, which gave the Soviets their excuses for "helping" socialist brothers in Hungary and Poland in 1956 and Czechoslovakia in 1967.
At other times, the dominant power can declare a sphere of influence without obtaining the consent of the smaller countries. The Monroe Doctrine was a unilateral declaration by the United States.
Currently, the idea of a sphere of influence is out of fashion in international circles.