Since Russian troops seized Ukraine’s strategic Crimean peninsula in late February, and separatists backed by Russian President Vladimir Putin began waging a bloody insurgency in the country’s east, the North Atlantic Treaty Organization has walked a fine line.

The transatlantic military alliance has sent hundreds of troops to Ukraine to train alongside Kiev’s forces. But at a major summit in early September, NATO declined to offer Ukraine membership. The alliance doesn’t really want to go to war over Ukraine.

If Russia were to expand its coercive campaign, however, and invade neighboring Estonia — where a security officer is said to have been abducted by Russian forces, a little more than a week ago — NATO’s 27 other member states would have little choice but to deploy troops in combat. They are obligated under Article 5 of NATO’s 1949 founding charter to defend each other from attack.

Yet it’s not at all clear that Russia would choose to fight on NATO’s terms. A Russian form of secret warfare called maskirovka might not qualify as a military attack — and could avoid triggering NATO intervention. Instead of fighting in the open, Russian troops entered Crimea in vehicles with license plates and insignia blacked out. There was no clear evidence — at least available publicly — that it was the Russian’s themselves taking action, rather than locals acting on their own behalf.

Such tactics stop short of open warfare and the nations thus invaded fear that if they counterattack, they would then be labeled the aggressors.

All the same, President Barack Obama visited Estonia in September to reassure that country and the other Baltic nations. “The defense of Tallinn and Riga and Vilnius,” Obama said in Tallinn on Sept. 3, “is just as important as the defense of Berlin and Paris and London.”

To that end, NATO has sophisticated plans to send progressively bigger and heavier forces to do battle on its frontier. The alliance could readily deploy more and better-armed troops than Russia.

Consider, the last time both Russia and NATO reported weapons totals, back in 2007: The alliance possessed a combined 61,000 tanks, armored vehicles, artillery, warplanes and helicopters against Russia’s 28,000. NATO’s active-duty military manpower exceeds 3 million. Russia has fewer than one million active troops.

Yet if Moscow did have designs on Estonia or another small NATO country bordering Russia, the attack wouldn’t necessarily come in the form of tanks and warplanes that NATO could easily identify and destroy.

Russia instead could do what it has done in Georgia, Crimea, eastern Ukraine and other former Soviet countries outside the NATO alliance. It could undermine them covertly before sending in a single soldier, a strategy Moscow has refined specifically to avoid a clear-cut military confrontation that it just might lose.

To be sure, a Russian invasion of Estonia or another NATO state would meet with overwhelming force from the United States, Canada and NATO’s European members. Alliance defense plans call for small, lightly armed units to act as “tripwires” — alerting commanders about the invading forces so that more numerous and heavily armed formations can rush in.

By themselves, the Baltic states are militarily weak. Between them, the three countries — which all joined NATO in 2004 — have just three outdated tanks and not a single frontline jet fighter. Bigger NATO countries take turns sending jets to Lithuania to patrol the three countries’ airspace.

But the alliance could reinforce Estonia or another small member state quickly. The U.S. Army’s 173rd Airborne Brigade in Italy stands ready to deploy on short notice. NATO also has a multinational reaction force that could deploy 14,000 troops in five days.

At the Wales summit, NATO Secretary General Anders Fogh Rasmussen promised to boost the readiness of 4,000 soldiers from the reaction force so that they can reach a battle zone in two days instead of five. “This force can travel light,” Rasmussen said, “but strike hard if needed.”

The United States possesses the bulk of NATO’s combat power. America keeps 67,000 troops in Europe on a permanent basis, along with 216 warplanes, according to the Stockholm International Peace Research Institute. The Pentagon also regularly practices methods of rushing additional forces to Europe by plane and ship.

Earlier this year, the U.S. Army moved a battalion-sized stockpile of tanks, armored fighting vehicles and self-propelled howitzers to Germany. Then during the Combined Resolve II exercise this summer, the American military flew in troops to practice activating the pre-deployed tanks in case of a crisis.

This fall, the Army is repeating the exercise with another battalion. It is also shipping in hundreds more tanks and armored vehicles — temporarily — for exercises in Poland and the Baltic states.

Within days of a Russian incursion into a country like Estonia, NATO could have hundreds of warplanes and thousands of soldiers ready to fight. Within a few months, the alliance could command thousands of warplanes and hundreds of thousands of troops — far exceeding any force Russia could hope to muster.

But Moscow is unlikely even to try matching NATO plane for plane or tank for tank. Instead, the Kremlin has developed a form of secret invasion it calls maskirovka. The strategy relies heavily on deception, deniability and special operations troops mixed with volunteer militias — armed with advanced weapons — to bring about political change outside Russia’s borders.

“The open use of forces — often under the guise of peacekeeping and crisis regulation — is resorted to only at a certain stage, primarily for the achievement of final success in the conflict,” General Valery Gerasimov, chief of Russia’s military general staff, wrote in an essay February for the weekly newspaper Military-Industrial Courier.

The strategy is tailor-made for creating confusion among defending armies trained to fight a conventional — and now largely obsolete — foe. Russia also relies on the heavy use of propaganda and information warfare to make the defending state appear to be an illegitimate occupier.

Washington’s eastern European allies are worried Russia could launch a stealth invasion without triggering Article 5 — which would necessitate a NATO response — because the operation’s initial stages might not legally constitute an attack by a foreign state.

“It is the operationalization of a new form of warfare that cannot be characterized as a military campaign in the classic sense of the term,” an April report from the National Defense Academy of Latvia’s Center for Security and Strategic Research warned. “The invisible military occupation cannot be considered an occupation by definition.”

One key reason for Obama’s European trip was to reassure Washington’s eastern European allies otherwise. “Article 5 is crystal clear,” Obama said on Sept. 3. “An attack on one is an attack on all.”

But it’s unclear whether NATO’s legal framework would count the introduction of ad-hoc Russian militants and armed civilians as an attack. Or whether Article 5 would only trigger on the arrival of conventional Russian military forces in the latter stages of the conflict. By then, it might be too late.

Russia’s strategy is also aimed at European politicians.

Though a conventional attack would bound these leaders by treaty to defend each other, an unconventional — and plausibly deniable — stealth invasion would provide just enough wiggle room for politicians who may rather avoid confronting Russia.

Some leaders could prefer to seek a political settlement. But this could be after the conditions on the ground have already changed in Russia’s favor.

NATO is able to defend against Russia’s tanks, but it might not be able to defend against maskirovka.

PHOTO (TOP): Canadian Air Task Force jets CF-18, which were flying air policing missions over the Baltic states, stand in the Siauliai air base, August 26, 2014. REUTERS/Ints Kalnins

PHOTO (INSERT 1): A freight car loaded with a self-propelled howitzer is seen at a railway station in Kamensk-Shakhtinsky, Rostov region, near the border with Ukraine, August 23, 2014. REUTERS/Alexander Demianchuk

PHOTO (INSERT 2): President Barack Obama joins in a meeting on the situation in Ukraine at the NATO Summit at the Celtic Manor Resort in Newport, Wales, September 4, 2014. REUTERS/Larry Downing

PHOTO (INSERT 3): NATO Secretary General Anders Fogh Rasmussen speaks at a news conference on the second day of the NATO summit at the Celtic Manor resort, near Newport, in Wales, September 5, 2014. REUTERS/Yves Herman

PHOTO (INSERT 4): U.S. 173 airborne brigade soldiers leave a C-17 aircraft during the “Steadfast Javelin II” military exercise in the Lielvarde air base, September 6, 2014. REUTERS/Ints Kalnins

PHOTO (INSERT 5): Dutch Brigadier-General Nico Tak, head of NATO’s crisis management center, at a news conference at Supreme Headquarters Allied Powers Europe (SHAPE), August 28, 2014.REUTERS/Yves Herman