Geopolitics is an interdisciplinary field studies, and is used to influence, of the relationships among human beings and their geography, their government, their history, and the natural world. The term was introduced by the German military officer and academic, Karl Haushofer, but acquired connotations of being a "Nazi science", and the term was deprecated, after World War II.
Haushofer and Germany
Haushofer retired from the German Army as a major general, and began teaching at the University of Munich in 1919. His essential core theory was that the state "is a biological organism which grows or contracts, and that in the struggle for space the strong countries take land from the weak." Influencing his thinking were the works of Oswald Spengler, Alexander Humboldt, Karl Ritter, Friedrich Ratzel, and Halford Mackinder. 
Rudolf Hess, one of his students, would soon introduce him to Adolf Hitler, and the biological theory certainly affected Hitler's writings on Lebensraum in Mein Kampf, and possibly the Nazi race and biological ideology in general.
The science that cannot speak its name
Studies of, and operational models for, global politics certainly continued, such as the containment policy described, internally to government, by George Kennan in 1946, in what has become known as the "Long Telegram"  Kennan, in 1947, wrote an article for the influential journal, Foreign Affairs, under the pseudonym "X". What became known as the "X Article", titled "The Sources of Soviet Conduct".
Henry Kissinger certainly had a geopolitical model in mind when moving to detente with the Soviets, and then the opening with China. Kissinger certainly used geopolitical arguments, going back to his senior thesis, "The Meaning of History: Reflections on Spengler, Toynbee and Kant." He continued his thoughts in a doctoral dissertation and book, A World Restored: Metternich, Castlereagh and the Problems of Peace 1812-22. It emphasized a balance of power system, which became the school of realism (foreign policy).
European federalism serves as a regional geopolitical model that affects the world.
The fall of the Soviet Union had immense geopolitical ramifications.
The term gained respectability in 1999, whe two British academics, Colin Gray and Geoffrey Sloan, published a scholarly investigation, Geopolitics, Geography and Strategy.
- Holger H. Herwig, The Daemon of Geopolitics: Karl Haushofer, and Adolf Hitler
- Karl Haushofer, Spartacus
- George Kennan (February 22, 1946), (Telegram) The Charge in the Soviet Union (Kennan) to the Secretary of State
- "X" (pseudonym for George Kennan) (July 1947), "The Sources of Soviet Conduct", Foreign Affairs
- Henry Kissinger (1973), A World Restored: Metternich, Castlereagh and the Problems of Peace, 1812-1822, Mariner Books