Napoleon and his army, the best in the world, couldn't take Russia. Hitler's army, probably the best in the world at the time, not only couldn't take Moscow, It couldn't get out of Russia alive. A three years war inside Russia was, and is, unthinkable.

Afghanistan has been called "the graveyard of empires". The same might be said today about Russia, or China, or the United States of America. You can nuke them back to the stone age, but conquer, as in occupy and usurp them? No. That's why diplomacy and mutually beneficial trade is necessary.