His writings served as welcome justification for imperial expansion.[15] Influenced by Mahan, Ratzel wrote of aspirations for German naval reach, agreeing that sea power was self-sustaining, as the profit from trade would pay for the merchant marine, unlike land power.[16] Haushofer was exposed to Ratzel, who was friends with Haushofer's father, a teacher of economic geography,[17] and would integrate Ratzel's ideas on the division between sea and land powers into his theories, saying that only a country with both could overcome this conflict.[18]