No one in Russia, NATO or the United States has gone that far yet. Still, the rhetoric and actions from both sides have definitely ratcheted up in recent days, raising concerns of a new arms race -- if not worse -- amid tensions both sides blame on each other.
The major players all claim their movements are defensive and necessary responses to their foe's provocation. None has talked of an invasion.
Still, that's not what some experts are worried about. They say a bigger fear is what things can happen, accidentally, when you have increasingly powerful military forces lined up so close to each other.
Part of it has to do with the unpredictable nature of other actors, like Russian-backed separatists in eastern Ukraine who may broaden their own conflicts by inadvertently or purposefully striking others. The biggest such example may be the 2014 shooting down of a Malaysia Airlines commercial plane over Ukraine by rebels.
Then there's the danger that something goes wrong as powerful militaries become more aggressive, as when a Russian fighter jet recently came within 10 feet of a U.S. Air Force reconnaissance aircraft over the Black Sea.
"Given the tempo of Russian military operations over the last year," said Steven Pifer of the Washington-based Brookings Institution think tank and a former U.S. ambassador to Ukraine, "you have more interactions, more possibilities for things to go wrong."
Military tit for tat
There's no doubt that the military tit-for-tat has picked up this week.
The U.S. Navy is among those participating in a NATO landing exercise in Sweden. Around the same time, U.S. Secretary of the Air Force Deborah James announced that it could be sending some of its most advanced warplanes to Europe in a show of force.
Already, the Pentagon has rotated B-2 and B-52 bombers, F-15Cs and A-10 attack planes as well as Army and Navy assets through Europe for exercises with allies under what's called Operation Atlantic Resolve. James said the F-22 Raptor, the Pentagon's premier fighter, could soon join them.
This is all in addition to previous U.S. military actions in support of Ukraine and several Baltic countries, some of whom fear Russian President Putin -- either directly or indirectly -- will come after them next.
In reaction, Russia's foreign ministry on Monday accused NATO countries of "sliding into a new military confrontation with destructive consequences."
That's not to say Moscow hasn't taken military action of its alone -- most conspicuously with its aircraft.
NATO announced late last year that it intercepted more than 400 Russian military planes in 2014 alone -- a 50% increase from the previous year.
Then there was the Russian jet that in May came within 10 feet of an American military plane in international airspace above the Black Sea.
"You don't have to fly 500 feet above a ship to do intelligence-gathering," Jorge Benitez, a NATO expert at the Atlantic Council, said. "(Putin) is trying to use these threats to push back on the West and say, 'I'm willing to do these things to get you out of my sphere of influence.'"
NATO decries 'nuclear saber-rattling'
Russian President Vladimir Putin has given little impression he'll back down.
"If someone threatens our territories, it means that we will have to aim our armed forces" toward the threat, he said Tuesday, according to state-run Sputnik news. "It is NATO that is coming to our borders, it's not like we are moving anywhere."
Putin upped the ante beyond provocative aerial maneuvers that same day, as he announced that he is buttressing his country's nuclear arsenal with an additional 40 intercontinental ballistic missiles.
In response, NATO Secretary-General Jens Stoltenberg criticized this "new nuclear capabilities and (increased use of) nuclear rhetoric," and warned, "The nuclear saber-rattling of Russia is unjustified, it's destabilizing and it's dangerous."
James, the U.S. Air Force's top civilian leader, called Russian activity in Europe the "biggest threat on my mind" when she noted that the U.S. is considering sending its top F-22 fighter jets to the region.
Russian officials, though, say they're the ones being threatened. That includes reports that the United States might deploy tanks and artillery to bases in Eastern Europe, a prospective move Russian Defense Ministry official Yuri Yakubov called "the most aggressive step by the Pentagon and NATO since the Cold War."
Defense Secretary Ashton Carter will announce that decision next week, sources told CNN.
Ukraine tension stirs rivalry
U.S. officials and observers point to the uprising in Ukraine as the start of the uptick in Moscow's confrontational behavior, which intensified amidst Western condemnation and pressure in the wake of Russia's seizure of Crimea.
President Barack Obama has looked to bolster Ukrainian military efforts through non-lethal aid and training, as well as give political support to Kiev and place pressure on the Russians through a bludgeon of sanctions coordinated with EU partners.
Obama has thus far resisted sending lethal aid to Ukraine -- an ally, but not a member of the NATO alliance.
U.S. and European leaders are already considering an additional round of sanctions they would impose on Moscow if it makes further military moves in Ukraine.
U.S. officials are particularly concerned by large-scale Russian military exercises near Ukraine set to take place this summer -- just the latest in a series of exercises that have come either close to the Ukrainian border or simulated military strikes, including nuclear strikes, on Europe.
Western officials believe that they need to respond in kind -- and that's come in the form of increased NATO military exercises with Baltic allies like Estonia and the latest American plans to send artillery to bases in the region.
"We want to make sure that NATO allies are defending their territory on a 24/7 basis and we'll continue to support them and exercise vigilance in that regard," White House Press Secretary Josh Earnest said Monday about increased U.S. military engagement in the region.
And while the U.S. looks to reassure Eastern European allies with these efforts, Russia's provocations continually unsettle those same countries, said Paul Stronski, who served as director for Russian policy issues on the National Security Council from 2012 to 2014.
"When the West talks about upping its presence in the region, this is their response," he said.
Nearly every American and European attempt at deterring Putin has instead triggered an opposite reaction: more military exercises, more provocative behavior and a persistent refusal to back down in the face of Western demands.
"I think we're going to be seeing more and more of this," said Benitez, who is not alone in arguing that the Obama administration is not doing enough to beat back Russian aggression. Republicans in Congress and others in Washington are calling for Obama to green light lethal aid to the Ukrainians.
Stronski, however, called for caution.
"It's time that people start to show some political restraint to rein this in on both sides," he said.
CORRECTION: An earlier version of this story gave the wrong title for Russian Defense Ministry official Yuri Yakubov.