For months, Moscow has supported separatist rebels in eastern Ukraine with weapons, supplies, cross-border artillery barrages, propaganda and political cover. The result has been a powerful and, in places, deeply popular insurgency that threatens Ukraine’s very existence in its current form.

Now Russian president Vladimir Putin is finally taking a more direct approach to exercising his country’s influence over its smaller, poorer western neighbor. Russian tanks and troops have attacked across the border near the Azov Sea, opening up another front against Kiev’s beleaguered army.

The Russian incursions have been small, subtle and slow, but despite Putin’s denials, they do amount to an invasion—ostensibly the final move in Moscow’s long-term strategy of destabilizing, dividing and defeating Ukraine.

The Kremlin’s maskirovka “masked war” strategy is working … and NATO—in theory the main protector of a free Europe—is scrambling to respond.

On Aug. 17, the German magazine Die Welt interviewed NATO Supreme Allied Commander Europe Philip Breedlove, an American Air Force general. Breedlove explained that NATO was ready to meet Moscow’s “little green men” in Ukraine with military force of its own.

The U.S. Army is sending more tanks to Eastern Europe. NATO has tripled air patrols over countries bordering Ukraine and Russia. The alliance is planning several large naval and ground exercises in the region.

But these moves represent an old way of thinking, harkening back to a time several decades ago when NATO could deploy ships, tanks and warplanes and realistically hope to deter Russian aggression.

That’s no longer the case. Old methods won’t stop Moscow’s maskirovka. Russia’s strategy is just indirect enough to sidestep traditional military and diplomatic processes that, in the past, might have allowed an opposing military alliances to meaningfully intervene—without triggering a global war that nobody wants.