Hitler later rejected the mountains as an adequate border, calling it absurd that "these middle-sized mountains" represented the boundary between the "European and Asiatic worlds", stating that one might as well accord that title to one of the large Russian rivers.[15] He explained that only a "living [racial] wall" of Aryan fighters would do as a frontier, and to keep a permanent state of war present in the east in order to "preserve the vitality of the race".