Sphere of influence
In the field of international relations, a sphere of influence (SOI) is a spatial/geographic region, or set of polities or nations, over which a leader nation state or other geopolitically powerful organisation has a level of cultural, economic, military, or political influence or exclusivity.
While there may be a formal alliance or other treaty obligations between the influenced and influencer, such formal arrangements are not necessary for a sphere to exist. Influence can be had through the use of hard as well as soft power. Similarly, a formal alliance does not necessarily mean that one country lies within another's sphere of influence. Historically, strong or strict exclusivities within spheres of influence, has often led to or been associated with higher levels of global conflict and tension between such spheres. Inter-sphere trade, interdependence and communication, on the other hand, have spawned periods of relative peace – as have periods where spheres of influence have been more weakly enforced and international associations more voluntary.
In extreme cases, a country or community within the "sphere of influence" of another, more powerful nation state, may become a client state or a functional subsidiary of that state -- serving in effect as a satellite state or de facto colony. This was the case, in the decades following the end of World War II, with the Soviet Union (through dominant power Russia) and smaller members of the Eastern Bloc (eg, the Baltic states; such power relations arguably exist in Belarus today). The system of spheres of influence, under which regions are divided amongst powerful nations, who then influence or intervene in the affairs of small nations in their region, continues to the present. Geopolitical power relations are often analyzed using notions of regional or global superpowers, great powers, and/or middle powers.
Sometimes different portions of a single country can fall into different spheres of influence, or a nation may fall into different spheres of influence depending on the factor being influenced (eg, trade, or cultural, influence). In the 19th century, the buffer states of Iran and Thailand, lying between the empires of Britain, France, and Russia, were divided between the spheres of influence of those three international powers. Likewise, after World War II, Nazi Germany was divided into four occupation zones, three of which later consolidated into West Germany and the remaining one became East Germany, the former a member of NATO and the latter a member of the Warsaw Pact.