Geopolitics (from Ancient Greek γῆ gê 'earth, land' and πολιτική politikḗ 'politics') is the study of the effects of Earth's geography on politics and international relations. Geopolitics usually refers to countries and relations between them; it may also focus on two other kinds of states: de facto independent states with limited international recognition and relations between sub-national geopolitical entities, such as the federated states that make up a federation, confederation, or a quasi-federal system. According to multiple researchers, the term is currently being used to describe a broad spectrum of concepts, in a general sense used as "a synonym for international political relations", but more specifically "to imply the global structure of such relations"; this usage builds on an "early-twentieth-century term for a pseudoscience of political geography" and other pseudoscientific theories of historical and geographic determinism.

At the level of international relations, geopolitics is a method of studying foreign policy to understand, explain, and predict international political behavior through geographical variables. These include area studies, climate, topography, demography, natural resources, and applied science of the region being evaluated.

Geopolitics focuses on political power linked to geographic space, in particular, territorial waters, land territory and wealth of natural resources, in correlation with diplomatic history, in particular the context of a larger power relative to its neighboring states of smaller or similar power. Some scholars have argued that geopolitics should serve as "an aid to statecraft." Topics of geopolitics include relations between the interests of international political actors focused within an area, a space, or a geographical element, relations which create a geopolitical system. Critical geopolitics deconstructs classical geopolitical theories, by showing their political or ideological functions for great powers. There are some works that discuss the geopolitics of renewable energy. The relationship between geopolitics and geoeconomics is often analyzed by two main schools of thought: the strategic school and the political-economic school.

The Austro-Hungarian historian Emil Reich (1854–1910) is considered to be the first having coined the term in English as early as 1902 and later published in England in 1904 in his book Foundations of Modern Europe.

Alfred Thayer Mahan (1840–1914) was a frequent commentator on world naval strategic and diplomatic affairs. Mahan believed that national greatness was inextricably associated with the sea—and particularly with its commercial use in peace and its control in war. Mahan's theoretical framework came from Antoine-Henri Jomini, and emphasized that strategic locations (such as choke points, canals, and coaling stations), as well as quantifiable levels of fighting power in a fleet, were conducive to control over the sea. He proposed six conditions required for a nation to have sea power:

Mahan distinguished a key region of the world in the Eurasian context, namely, the Central Zone of Asia lying between 30° and 40° north and stretching from Asia Minor to Japan. In this zone independent countries still survived – Turkey, Persia, Afghanistan, China, and Japan. Mahan regarded those countries, located between Britain and Russia, as if between "Scylla and Charybdis". Of the two monsters – Britain and Russia – it was the latter that Mahan considered more threatening to the fate of Central Asia. Mahan was impressed by Russia's transcontinental size and strategically favorable position for southward expansion. Therefore, he found it necessary for the Anglo-Saxon "sea power" to resist Russia.

Nicholas J. Spykman was both a follower and critic of geostrategists Alfred Mahan, and Halford Mackinder. His work was based on assumptions similar to Mackinder's, including the unity of world politics and the world sea. He extends this to include the unity of the air. Spykman adopts Mackinder's divisions of the world, renaming some;

Under Spykman's theory, a Rimland separates the Heartland from ports that are usable throughout the year (that is, not frozen up during winter). Spykman suggested this required that attempts by Heartland nations (particularly Russia) to conquer ports in the Rimland must be prevented. Spykman modified Mackinder's formula on the relationship between the Heartland and the Rimland (or the inner crescent), claiming that "Who controls the rimland rules Eurasia. Who rules Eurasia controls the destinies of the world." Spykman died before the end of World War II, but his theories influenced the concept of containment, in that Spykman argued that the United States should support states in the Rimland to balance against a Heartland power (see also Truman Doctrine).