There are times of small, peripheral wars, which big countries fought against small ones, and there are times of big, central wars, on which big countries fight each other to eliminate each other or determine laws of the post-war peace. Big countries don't want to actually big wars, but they want to achieve their certain goals. And when one side violate the rules of post-war peace and officially declare its unwillingless to accept the interests of another side (as it was with US aggressions against Serbia and Iraq) the next big war becomes inevitable (to establish the new rules).
But the contradictions between Russia, China, Europe, India and America in the missile age can be solved without invasions in smaller countries. One side nuke another's sides strategic forces and then suggest mutually acceptable terms of post-war peace. If they agree - the WWIII is over and survived big countries start their preparations for WWIV.