U.S. Defense Secretary Ash Carter urged Russia to stop "going backwards" towards the Cold War era with "saber-rattling" and aggressive actions that have put NATO on high alert.
"We do not seek to make Russia an enemy," Carter said on May 3 at a ceremony in Stuttgart to install U.S. Army General Curtis "Mike" Scaparrotti as NATO's new top commander in Europe.
"We do not seek a cold, let alone a hot war with Russia. But make no mistake: We will defend our allies, the rules-based international order, and the positive future it affords us."
Carter cited Russia's intervention in eastern Ukraine, its annexation of Crimea in 2014, and what he called Moscow's efforts to intimidate its Baltic neighbors, which are NATO members that the United States is sworn to defend.
"Most disturbing" is Russian rhetoric about using nuclear weapons, he said.
"Moscow's nuclear saber-rattling raises troubling questions about Russia's leaders' commitment to strategic stability, their respect for norms against the use of nuclear weapons," he said.
His comments came as Estonian Defense Minister Hannes Hanso disclosed that Russian military jets have been operating in an "incredibly reckless" manner in Baltic airspace.
Not only do they regularly violate Estonian airspace, he said they often fly across the Baltic Sea with their transponders switched off so they cannot be detected by civilian radar.
"It is incredibly reckless and it is an accident waiting to happen," Hanso said. "Imagine a collision between a Russian plane and a civilian plane. Normal countries don't do this sort of thing. So it is just to provoke and to challenge us. It is unacceptable."
The United States and NATO's European members have responded to what they see as an increased threat from Russia by rotating more troops into eastern member states and imposing economic sanctions over Russia's actions in Ukraine.
But Carter seemed to concede that those measures haven't had the desired effect of making Russia back off. Rather they have led to steadily deteriorating relations between Russia and the West that he said he regrets.
"We haven't had to prioritize deterrence on NATO's eastern flank for the past 25 years. But while I wish it were otherwise, now we have to," he said.
Carter said he continues to hold out hope that Russia will eventually abandon its confrontational approach.
"The United States will continue to hold out the possibility that Russia will assume the role of a constructive partner moving forward, not isolated and going backward in time as it appears to be today," he said.
"Much of the progress we've made together since the end of the Cold War, we accomplished with Russia. Let me repeat that. Not in spite of Russia, not against Russia, not without Russia, but with it."
Carter made no mention of two NATO moves that many believe prompted, at least in part, Russia's turn away from the West: the expansion of NATO toward Russia's western border and U.S. placement of missile defenses in Europe.
"We'll keep the door open for Russia," he said. But it's up to the Kremlin to decide."
In contrast to Carter, outgoing NATO commander General Philip Breedlove appeared to have a more positive view of Russia's current leadership.
Breedlove described Russian President Vladimir Putin as "more reasonable" than other possible alternatives in Russian leadership circles, in an interview with the Wall Street Journal on May 2.
"We have a very strong leader who is surrounded by a small group of people who make the decisions. Most of those decisions are focused on the preservation of the regime. We believe that that group will be with us for some time," Breedlove said.
"And we're not sure that we might be in a better place if we saw a change," he said. "We are unsure that we would be in a better place if Mr. Putin was not there. He may be a more reasonable voice in the middle of that group," he said.
With reporting by AP, Reuters, TASS, and Wall Street Journal