According to a poem written by a Victorian era Britbong;

Who rules East Europe commands the Heartland;
who rules the Heartland commands the World-Island;
who rules the World-Island commands the world.

—Mackinder, Democratic Ideals and Reality, p. 106

That means that if Russia makes friends with China and conquers Ukraine, then the God-Emperor Putin will rule the entire world from his Holy Orthodox throne in the Capitol of the Third Rome. At that's just not a world that the Lands of the Outer or Insular Crescent can tolerate.

https://en.wikipedia.org/wiki/The_Geographical_Pivot_of_History