Germany–Russia relations display cyclical patterns, moving back and forth from cooperation and alliance to strain and to total warfare. Historian John Wheeler-Bennett says that since the 1740s:Relations between Russia and Germany...have been a series of alienations, distinguished for their bitterness, and of rapprochements, remarkable for their warmth....A cardinal factor in the relationship has been the existence of an independent Poland...when separated by a buffer state the two great Powers of eastern Europe have been friendly, whereas a contiguity of frontiers has bred hostility.