"We do not seek to make Russia an enemy. But make no mistake – we will defend our allies, the rules-based international order, and the positive future it affords us," Carter said at the U.S. military's European Command in Stuttgart, Germany.
Carter expressed a desire not to start a new Cold War with Russia – or a "hot" one.
But he said Russia seeks to "erode" the peaceful order Europe and the rest of the West have enjoyed since the end of the Cold War.
Carter, in vowing the U.S. would defend its allies, warned Russia is increasing its submarine patrols to the North Atlantic. He did not specifically mention the recent "barrel rolls" by Russian jets over U.S. military aircraft in the past few weeks but accused Russia's leaders of "nuclear saber-rattling" and putting the world at risk in the process.