NATO and Russia watch one another closely in Eastern Europe
Sweden scrambled fighter jets to intercept two Russian military planes that flew too close to Swedish airspace.
With Russia flexing its muscles, three of its Baltic neighbors -- Estonia, Latvia and Lithuania have asked NATO to permanently deploy ground troops as a deterrent.
On Europe's Eastern frontier, NATO F-16s and Eurofighters drill for something they're doing more and more, intercepting Russian military aircraft flying too close for comfort to European airspace.
A cockpit video shows NATO jets shadowing Russian planes, which often try to stay invisible by turning off their transponders.
We watched the NATO pilots practice from a military transport plane. But last years in the Baltic states, they did this for real more than 150 times, a nearly four-fold increase on 2013.
If the NATO fighter jets don't intercept and identify the Russian military aircraft, the fear is they could cause a crash with a commercial airliner.
With its dangerous tactics, Russia seems to be probing NATO's air defenses, and testing how the West will react.
"Let's put it this way," said General Philip Breedlove, the Supreme Commander of NATO in Europe. "It's certainly not the kind of behavior you would see between two partners."
And nor is this. In Ukraine last year, Russia seized Crimea and gave its military backing to armed rebels in the east of the country.
The U.S. accuses them of shooting down a Malaysian airliner in July.
Lithuanian Captain Ieva Gulbiniene told us many here fear their former Soviet overlords.
"Occupation - this is what they are afraid of," she said. "Nobody wants to go back to the ages what was 25 years ago."
If Russia did what it's done in Ukraine to a NATO ally in Eastern Europe, how would NATO respond?
"NATO is committed to defending its allies," said Breedlove. "We will defend any NATO nation that's attacked by any nation."