During World War I, the British blockade of trade to Germany caused food shortages in Germany and resources from Germany's African colonies were unable to help; this caused support to rise during the war for a Lebensraum that would expand Germany eastward into Russia to gain control of resources to stop the food shortages.[14] In the period between the First and the Second World Wars (1919–39) German nationalists adopted the term Lebensraum to their politics for the establishment of a Germanic colonial-empire like the British Empire, the French Empire, and the empire that the U.S. established with the westward expansion of the "American frontier", which was advocated and justified by the ideology of Manifest Destiny (1845).[15] Ratzel said that the development of a people into a society was primarily influenced by their geographic situation (habitat), and that a society who successfully adapted to one geographic territory would naturally and logically expand the boundaries of their nation into another territory.[3] Yet, to resolve German overpopulation, Ratzel said that Imperial Germany (1871–1918) required overseas colonies to which surplus Germans ought to emigrate.[16]