The big topic of discussion at NATO's biannual summit in Warsaw Friday and Saturday is how the Alliance ought to respond to a newly aggressive Russia.

Another way to put it: A2AD, that "terrible acronym,” in the words of Douglas Lute, the U.S. Ambassador to NATO. A2AD stands for Anti-Access Area Denial, and it captures the security and political challenge posed by Moscow's military investments along NATO's frontiers.

Over the past few years, Russia has deployed sophisticated anti-air and anti-ship defenses, bombers and missiles at key locations, most significantly for NATO in the enclave of Kaliningrad that's sandwiched between Poland and Lithuania. These A2AD systems are intended to "deny" enemy forces "access" to these regions.

The Russian push to strengthen A2AD at its single Mediterranean port, at Tartus in Syria, in the Crimean peninsula annexed from Ukraine in 2014 and in the Baltic and Barents Sea basins caught NATO off guard. In particular, the deployment in Kaliningrad, which includes Iskander missiles that could be converted into tactical nuclear weapons, calls into question the Alliance's ability to defend the three Baltic states and Poland in case of hostilities with Russia.

“A2AD is the new chess game that we are facing between the West and Russia," said Fabrice Pothier, who until recently was head of policy planning at NATO. "Like during the Cold War, the theater of this game is Europe.”

While there's little disagreement in NATO over Russia's military investments, the allies are split over what to do about it. The debate over A2AD mirrors the one broadly over Russia in the wake of Vladimir Putin's attack on Ukraine in 2014. The Baltic countries and Poland favor a robust response, calling for permanent NATO bases on their territory and more investment in common defense. The U.S. and U.K. fall in the middle, recognizing the threat and calling for "a multi-pillar approach," short of going as far as the Eastern Europeans want. And Germany and Italy, historically friendlier with Russia, want to cool down the rhetoric and avoid anything that might escalate tensions with Moscow.

Commanders at war

Russia's push to boost defenses along NATO's frontier has a potentially double benefit from Moscow's point of view: To box in the Alliance military, and to sow political divisions. Just as NATO won the Cold War without firing a shot, said a senior NATO official, Putin is hoping to “paralyze the Alliance to an extent that politically it’s defeated.”

The argument has been running within NATO since the Ukraine conflict. Ahead of the Warsaw summit, General Philip Breedlove, who retired this spring as NATO's supreme allied commander, reopened it, calling on the Alliance to consider acquiring "independent A2AD capabilities."

"NATO is in urgent need of more long-range, survivable precision strike capabilities from the ground since it is currently almost completely dependent on air forces and aviation forces to attack A2AD," he said last week in Washington.

The retired American general was nicknamed "Dr. Breedlove" (after the Cold War film classic Dr. Strangelove) by detractors inside NATO, above all the Germans, who considered his public posture toward Russia too aggressive.

Other commanders had raised the alarm too. Frank Gorenc, commander of U.S. Air Forces in Europe, last year warned that Russia had "closed the [A2AD] gap" and that Kaliningrad-based missiles "could shut down NATO reinforcements to the Baltics in a crisis."

The outspokenness didn't go down well with other allies. The French General Denis Mercier, who's currently the supreme allied commander for transformation at NATO, is one of numerous officials to push back against the Russia hawks. "A2AD capability is not offensive or aggressive in nature," he said last October. "It’s principally a defensive measure. So we have to consider it, we have to be aware of it, we have to include it in our planning but it’s not the threat as such."

'Quasi-open war'

Still, recent "crisis exercises" at NATO have highlighted the vulnerability. A war game in March put on by NATO, with Finland and Sweden invited as observers, simulated "fictitious but realistic" hostilities, according to a NATO official who took part. Though geographical names are never used, one took place clearly in the Suwałki Gap, the land route from Poland to Lithuania that abuts Kaliningrad. If Russia seized control of this region, the Baltic countries would be physically cut off from the rest of the Alliance. "We realized that ... we were facing a significant challenge to penetrating certain areas," the NATO official said.

Separately, a recent war games study conducted by the U.S.-based Rand Corp. found Russian troops could take Estonia and Latvia within 36 to 60 hours. The Rand study concluded there are not enough NATO troops in the region, and that existing forces are not well-positioned to stop Russian mechanized units from storming across the border.

"NATO does not realize that it is losing ground, power and legitimacy since it fails to comply with its core premise as defensive alliance. We need to boost our presence," said an ambassador to NATO from a member country.

Officials say France, Germany and the U.K. are reluctant to respond to Russia's A2AD buildup blow-by-blow. "Now the burden of escalation is on us and to deal with the Russian A2Ad we would have to go to such a level of escalation that we would get into a quasi-open war," a senior NATO official said.

As a way to deter Russia, NATO countries in Warsaw will approve the deployment of a four multinational battalions that will rotate through Estonia, Latvia, Lithuania and Poland. "Allies will also commit to deliver heavy and high-end forces and that those forces will be fully trained and exercised," said Oana Lungescu, NATO spokesperson. "We will ensure that we have the full range of capabilities to counter the full spectrum of challenges we face, including A2AD."

Russia denies it is trying to reclaim military control over its former sphere of influence in the Baltics. "Russia is not moving," said Alexander Grushko, Russia’s ambassador to NATO. "We stay where we stayed. It is NATO that is moving its military infrastructure closer to Russian borders and deploying additional military assets."

Grushko added Moscow retains the right to protect itself in this "new security environment, also in view of the Warsaw summit."