On Monday, NATO Commander Gen. Philip Breedlove had a not-so-subtle message for Russia: it will consider stealth and unofficial invasions to be a trigger for war.

Europe knows these stealth invasions all too well. Russia first sent teams of special operations forces wearing uniforms without insignia in February into Crimea and then later into eastern Ukraine to work with Russian minorities inside the country to begin an insurrection. Ukraine’s military has been fighting these “little green men” ever since. But until recently, Russia has not even acknowledged sending anyone into the country.

Breedlove, speaking at the Atlantic Council on Monday, said if the Kremlin tried that in one of the NATO allies that border Russia—like the former Soviet republics in the Batlics, for example—it would risk triggering Article Five of NATO’s charter which is the section that calls on the alliance to come to the defense of a member state being attacked.

It’s hardly a hypothetical issue. Over the weekend, a Russian foreign ministry official said violations of the rights of Russian speakers in the Baltic states would have “far reaching, unfortunate” consequences, according to a report in Buzzfeed. The message was seen as a threat from Moscow to meddle further in the internal affairs of the three states that were once part of the Soviet Union.

“We have to look at those forces in our border nations where there are substantial Russian populations and how do we better prepare this initial onslaught of this hybrid war,” he Breedlove said. “How do we better prepare our allies to characterize, understand and survive the initial onslaught of the little green men scenario?”

Breedlove added that the issue was discussed this month at Wales at the head of state summit. “We had great acceptance among the NATO allies though that if you attribute this ‘little green men’ issue to an aggressor nation, it is an Article Five action and then all of the assets of NATO come to bear.”

How those assets will come to bear is still being worked out. Breedlove said his expectations were surpassed at this month’s NATO summit when the alliance agreed to create a rapid reaction force. But the details of that force are still being discussed.

Breedlove said he wanted NATO to also create a new command headquarters that would only focus on responding to threats to NATO members and how to respond quickly.

He also said he hoped the deployments of NATO forces in countries on Russia’s borders would be able to prepare the groundwork for the rapid insertion of combat troops in a worst case scenario. Those details will be discussed later this month in Vilnius, Lithuania at a meeting of NATO’s military committee, Breedlove said.

Breedlove himself said that he was “beginning to see” a similar kind of stealth or hybrid invasion in Moldova, which is not a member of NATO, as well. “We have seen the script play out in Crimea, we have seen the script play out in eastern Ukraine and we are beginning to see the script play out in Moldova,” he added.

Russia watchers have been warning since the spring that Moldova could be Putin’s next target. With already 2,500 peacekeepers stationed in the breakaway Moldovan province of Transnistria, Russia has a natural base of operations from which to begin destabilize the country’s pro-Western government.

Hannah Thoburn, a Eurasia analyst at the Foreign Policy Initiative, said Breedlove’s warning about hybrid invasions and little green men was significant. “He is saying the West needs to think along the gray lines that Russia likes to fight within. We need to move our benchmark for what is an invasion from waiting for a full column of tanks to cross a border to this new hybrid warfare,” she said.