Wikipedia
Geopolitik is the branch of uniquely German geostrategy. It developed as a distinct strain of thought after Otto von Bismarck's unification of the German states but began its development in earnest only under Emperor Wilhelm II. Central concepts concerning the German race regarding economic space demonstrate continuity from the German Empire toto Adolf Hitler's Third Reich. However, imperial geostrategist, German geopoliticians, and Nazi strategists did not have extensive contacts with one another, suggesting that German geopolitik was not copied or passed on to successive generations but perhaps reflected the more permanent aspects of German geography, political geography, and cultural geography.
It developed from widely varied sources, including the writings of Oswald Spengler, Alexander Humboldt, Karl Ritter, Friedrich Ratzel, Rudolf Kjellén, and Karl Haushofer. It was eventually adapted to accommodate the ideology of Hitler.
Its defining characteristic is the inclusion of organic state theory, informed by social Darwinism. It was characterized by clash of civilizations-style theorizing. It is perhaps the closest of any school of geostrategy to a purely nationalistic conception of geostrategy, which ended up masking other more universal elements.
Germany acted as a revisionist state within the international system during both World Wars by attempting to overthrow British domination, and counter what it saw as rising US and Russian hegemony. As a latecomer to nationhood proper, lacking colonies or markets for industrial output but also experiencing rapid population growth, Germany desired a more equitable distribution of wealth and territory within the international system. Some modern scholars have begun to treat the two World Wars caused by Germany as a single war in which the revisionist Germany attempted to bid for hegemonic control with which to reorder the international system.
German foreign policy was largely consistent in both wars. The Nazi foreign policy was unique insofar as it learned from what it saw as past imperial mistakes but essentially followed the very same designs laid out by German geopolitik and the historical record of the empire.