In both Crimea and the subsequent fighting in the Donbas region of Ukraine, Russia’s signature tactic has been the use of so-called “Green Men,” soldiers without identifying insignia whose identity as Russian soldiers the Kremlin denied. Ukraine, Georgia, and even NATO members like Estonia now fear that they could be the next target for Russia’s Green Men. NATO, alarmed by the need to prepare for this unexpected tactic, has committed to develop new countermeasures to defend against this threat. Green Men, or deniable forces, are a central part of what has come to be called “hybrid warfare” in the “gray zone” between war and peace. All of this seems to be a new and innovative departure from traditional tactics, perhaps even a new model for conflict in the 21st century.

However, deniable forces are nothing new. Nor, in fact, is the specific phenomenon of using them to seize a piece of territory, as Russia did in Crimea. There is a long history of hybrid warfare in general and of intervening with deniable forces in particular. This history points not just to the enduring nature of the threat, but also to the contours of a “counter-hybrid” strategy to defeat it.

In the course of a broader research project for which I compiled data on every land grab since 1918, 105 land grabs in total, I found three instances before Crimea of deniable forces seizing territory. In 1999, Pakistani forces crossed the Line of Control in the Kargil region of Kashmir, occupying positions overlooking strategically important roads in Indian territory. Like the Russians, Pakistan used deniable forces that they described as Kashmiri insurgents. Unlike the Ukrainians, the Indians counterattacked, absorbing heavy casualties to expel the Pakistanis.

Pakistan did not invent this gambit. On the contrary, these tactics predate Pakistan. In 1932, armed Peruvians dressed as civilians seized the remote border town of Leticia. As Colombia amassed the forces needed to remove the invaders, the Peruvian military intervened openly to back them. Colombia then dispatched a flotilla of gunboats that journeyed through the Atlantic and all the way up the Amazon River to engage the Peruvians, retaking part of the territory and recouping the rest through negotiation.

Russia has even been the victim of a land grab by deniable forces. In 1919, Finnish volunteers acting ostensibly on their own initiative (but in actuality with government sanction) invaded parts of Russian Karelia with Finnish populations, hoping to annex them to Finland. This Finnish attempt to exploit the political disorder of the Russian Civil War was short-lived. The Red Army was able to muster the forces to repulse the Finns.

Deploying deniable forces to intervene in an ongoing war, as Russia has done during the fighting in Donetsk and Luhansk, is yet more common. Even without deceiving informed observers, Moscow seems to believe that a semblance of deniability offers some hope of minimizing the repercussions of aggression. This is not a new assumption for those deploying deniable forces. During the Korean War, Russian pilots flew Russian aircraft in combat against the U.S. Air Force under the guise that they and their planes were Chinese. This continued even after radio chatter in fluent Russian confirmed American suspicions about the pilots’ identities. Mussolini’s Italy dispatched tens of thousands of soldiers without their normal identifying insignia as “volunteers” to intervene on behalf of the Fascists in the Spanish Civil War. The Italian Navy even launched attacks using submarines in lieu of surface warships in order to avoid culpability for their actions, in effect deploying submarines as deniable forces at sea. These are time-honored tactics.

The anxiety about hybrid warfare as a novel and innovative form of aggression is misplaced, but a threat need not be new to be dangerous. For those in the Baltic and elsewhere worried that Russia will again employ the tactics used in Crimea, however, it is perhaps encouraging that Russia’s success at seizing territory with deniable forces is atypical. Russia succeeded in Crimea with the same tactics that failed Pakistan in Kargil, Peru in Leticia, and Finland in Karelia, for an overall record of one success to three failures since 1918.

In each instance, the defender countered hybrid tactics in the same way. They accepted the fictitious terms of the conflict and mobilized enough strength to defeat the deniable forces on the battlefield. They sought to engage the deniable forces without also attacking any uniformed forces of the aggressor or striking targets in the aggressor’s territory, keeping the fighting contained. On the battlefield (although not in their rhetoric), both sides maintained the fiction that the conflict was something less than an open attack by the aggressor. India was extremely careful in this regard as it retook Kargil, even avoiding airstrikes that would require planes to overfly Pakistan’s side of the Line of Control.

In each case, the defender amassed enough military force to defeat the invading deniable force in a stand-up fight. This compelled the aggressor to make a difficult choice: intervene openly to salvage the operation (as Peru attempted) or accept that the gambit has failed and withdraw the deniable forces (as Pakistan and Finland elected). The objective of the defenders was to force the aggressor’s hand, obliging them to choose withdrawal.

There is an irreducible risk to this counter-hybrid strategy, because it can induce the aggressor to double-down and intervene openly. However, there is also risk in letting aggression go unchallenged. The choice of whether to confront hybrid tactics belongs to the nation under attack. If the choice is to resist, this counter-hybrid strategy has been the way to fight back.

Asked how he would counter “green men” crossing the border from Russia, Riho Terras, Estonia’s top general, gave a simple answer, “You shoot the first one to appear.” Although this statement may first appear to be the bombastic rhetoric of a reckless David insufficiently awed by the Russian Goliath, this approach is in fact supported by both the most relevant historical precedents and a calculated strategic logic.

For this counter-hybrid strategy to succeed, the defender must have enough military strength to be able to prevail in the initial, limited conflict fought under the guise of a purely internal armed struggle. This is why Ukraine has had so much trouble with hybrid warfare. Until Ukraine can strengthen its forces to the extent that Russia would need to intervene beyond the breaking point of an already threadbare veil of deniability in order to prevail (for instance, with its air force), Kiev will continue to struggle to defeat hybrid warfare. And it is Ukrainian capabilities that matter most, not those of the United States or NATO. Due to the desirability of maintaining the pretense of an internal conflict, defeating future deniable forces is a task best suited to the military of the state under attack, not its allies.

Ukraine, Georgia, Estonia, and other Russian neighbors will never be strong enough to defeat the Russian military on their own, but that is the wrong barometer. These countries need only be able to defeat a force small enough that Russia can deny responsibility for it. Taking that option off the table leaves Russia only the options of open war and acquiescing to the status quo. This amounts to the classic deterrence problem, with the added threat of hybrid warfare nullified.

Dan Altman is a U.S. Foreign Policy and International Security Postdoctoral Fellow at the Dickey Center for International Understanding at Dartmouth College.

Photo credit: Anton Holoborodko