The U.S. secretary of defense had some harsh words for Russia, accusing Moscow of "clear ambition to erode the principled international order."
Ash Carter made the comments on September 7 in a speech at Oxford University in England.
He told students Russia had engaged in "unprofessional behavior" in Ukraine, Syria, and cyberspace, and said Moscow was also guilty of nuclear "saber rattling."
"We don't seek an enemy in Russia. But make no mistake: we will defend our allies, the principled international order, and the positive future it affords all of us," Carter said.
"We will counter attempts to undermine our collective security. And we will not ignore attempts to interfere with our democratic processes," he said.
Carter said the United States and Russia made progress to cooperate after the Cold War, but he said now Russia "appears driven by misguided ambition and misplaced fear."