I have read this was the cause of both World War 1 and World War 2. Russia has now seized the Crimea and followed that up with a War in Eastern Ukraine. Does this have any relevance today?
This relates to the so-called Heartland Theory by people like Britain's HJ MacKinder. Whoever controls East Europe dominates the "Heartland" (which also includes Germany and Russia) of World Island (the Eurasian land mass). Whoever controls the Heartland dominates World Island. And whoever controls World Island dominates the world.
Therefore, any forceful change of the balance of power in East Europe will likely have global domino effects (maybe not World War III).