They had over a million soldiers on the borders months ago (right after the afghanistan conflict) ... And nothing happened , cuz the so much contries pressured them not to do so ... Even if no body took action I bet they would never start a new war , since they're both nuclear now ... These people ain't too dumb.
What is not politicaly correct is the U.S. intentional ignoring of the UN and the international comunity for their own plans.
If they moved on like this ignoring countries such as Russia , China (France & Germany currently too), and destroyed the UN authority , this is what may lead to the world war III .
