The Ukraine crisis has seen an interesting phenomenon: the appearance in Crimea and eastern Ukraine of seemingly professional soldiers in Russia-style combat uniforms with Russian weapons but without identifying insignia. Ukrainians coined the term “little green men” when such soldiers first manned roadblocks and seized strategic points on the Crimean peninsula.

NATO should think through what this could portend for Alliance security, specifically, how to react if little green men appeared in Estonia or Latvia.

The little green men turned up in Crimea at the end of February. When asked at a March 4 press conference about the soldiers and their Russian-style combat fatigues, President Vladimir Putin denied that they were Russian, calling them “local self-defense units.”

The truth soon came out. On March 28, Putin congratulated Russian officers in the Kremlin for their conduct of the Crimean operation. The Russian Defense Ministry issued a victory medal for the “return of Crimea.” In a May 18 telethon, Mr. Putin acknowledged that the troops in Crimea were Russian.

In the same telethon, the Russian president denied that the soldiers who in April had begun seizing buildings in eastern Ukraine were Russian. While many of the armed separatists in Donetsk and Luhansk undoubtedly are local, some have looked and handled themselves a lot like the little green men/Russian soldiers who so efficiently took control of Crimea. Will Mr. Putin at some later date confirm that Russian personnel have in fact been involved in eastern Ukraine?

Would Putin Challenge NATO Security and Solidarity?

Whether he does or not, NATO should think through the implications of the little green men phenomenon, in particular for the Baltic states of Estonia and Latvia. Ethnic Russians in each of those countries comprise about one-quarter of the population. While most seem content — relatively few ethnic Russians have chosen over the past 20 years to depart those countries and return to Russia — many do not hold citizenship.

This may not be a high-likelihood event, but it certainly is not a zero probability. Moscow regularly complains about the treatment of ethnic Russians in the Baltics, and Putin has asserted a right to intervene in other states to defend ethnic Russians and Russian-speakers, regardless of their nationality.

Putin, moreover, openly displays a strong sense of grievance against NATO. He might find attractive the idea of a plausible-denial operation that could challenge the security of a NATO member and undermine Alliance solidarity and credibility. The relatively limited investment that Moscow has made in fueling armed separatism has certainly sewn chaos in eastern Ukraine.

NATO members should be discussing now how the Alliance would respond if, say, soldiers without insignia assisted by locals were to seize a local administration building in eastern Estonia, just across the border from Russia. The first question would be whether NATO would consider this an internal problem for Estonia to handle itself or treat it as an Article 5 contingency, requiring that allies consider it an attack on all.

Preventing Contagion

Alliance members likely would not come to a firm decision on how to respond absent a real situation. Still, a preliminary discussion of the considerations that would guide a decision would be useful. That could speed a NATO decision in a real-world crisis. A fast response would be important to quickly contain (and end) the seizure, before things degenerated and seizures spread to other locations, as happened in eastern Ukraine.

This will not be an easy discussion. Some allies could well take the position that, absent compelling evidence of Russian involvement, such a crisis would be an internal security matter, to be addressed by Estonia’s own police and security forces. That might disappoint the Estonian government, but it would be useful for Tallinn to know.

If, on the other hand, NATO allies were inclined to consider this kind of case as having Article 5 implications, the Alliance should discuss what response would make sense and plan accordingly. That includes how NATO might assist Estonian security forces.

One option that Ukrainian security forces apparently lacked the capacity for, particularly after local police force structures essentially collapsed, would be a police operation backed by special forces. The point would be to isolate the seized building; move possible crowds of demonstrators away from the area; cut electricity, gas, water and sewage connections; and then wait out the occupiers.

Local forces in Estonia (or Latvia) might well be capable of carrying out such an operation on their own. But NATO pre-planning and involvement would augment their capabilities. It would also send important political signals: first, a message of support to governments that could find themselves victims of a little green men attack, and second, a message to the Kremlin that the tactic would not work against a NATO member.

This article was originally published in Spiegel Online.