Russia's aggression in Europe — its invasion of Ukraine, its military flights up the noses of NATO states, its nuclear saber rattling — has faded from the news. But it's still very much a threat, which is why the US is planning to quadruple its military spending in Europe, something NATO's European members have welcomed, to deter Russia.

In other words, the dynamics that brought Cold War–style military tension to Europe in 2015 are still with us. And that tension can be dangerous.

This summer, I wrote about a small but alarmed community of analysts and experts in the US, Europe, and Russia who earnestly worried that the risk of an unintended war had grown unacceptably high. A survey of 100 policy experts yielded an aggregate assessment of 11 percent odds of war and 18 percent odds that such a war would include nuclear weapons. (A subsequent, larger survey backed this up.)

Since then, my informal check-ins with my sources have led me to believe that this concern has not dissipated. And, in late January, scholars with the Zurich-based Center for Security Studies produced a map, as part of a longer report that you can read here, that helps show why this is still a real issue:

The map shows military exercises held by Russia and by NATO in 2014 and 2015. Each circle represents an exercise, and the larger circles mean more troops participated.

Obviously, both Russia and European states have been holding military exercises since before tensions spiked last year. And that's exactly the point: Military volatility is baked into Eastern Europe, such that when tensions do spike it has the capability to make the continent suddenly much more precarious.

I want to call your attention to the Baltic Sea, the body of water tucked between Sweden and the NATO-allied Baltic states of Estonia, Latvia, and Lithuania. (Recall that all NATO members, including the US, have pledged to go to war to defend any other NATO member that is attacked.) That little region is the focus of all this.

The geography of the Baltics make it enormously insecure for both Russia and for NATO, and this is why nearly every expert I spoke to warned that it is a potential tinderbox, where some unforeseen accident, miscalculation, or provocation could, in an unlikely but real worst-case scenario, send both sides careening into a conflict that neither wants.

The Kaliningrad problem

Look at the little red spot sandwiched between Poland and Lithuania: That's a part of Russia called Kaliningrad, which Russia took over after World War II. Kaliningrad is heavily militarized, partly because Russia uses it as a base for projecting power and partly because Moscow's conspiratorial-minded leaders fear that Europe is bent on retaking Kaliningrad.

Russia worries that in the case of any conflict small or large, Europe and the US would exploit it as an excuse to seize and pacify Kaliningrad. (If that sounds crazy to you, it doesn't to Russian leaders, who after all just seized and annexed part of a foreign country, and who earnestly believe the US is bent on Russia's destruction.) So it has built up Kaliningrad's defenses.

But Kaliningrad is completely isolated from the rest of Russia; it's surrounded by NATO states. And after Ukraine, NATO began putting a lot more troops and tanks in those NATO states. This was meant to defend the Baltics from a possible Ukraine-style provocation, but it also ended up cutting off Kaliningrad even more. There's other stuff happening as well — for example, the Baltics are moving onto a separate power grid, which could make Kaliningrad more reliant on Europe to power itself.

Russia clearly feels it needs a plan to defend Kaliningrad in the case of a conflict. So it's done two things, which are likely meant as defensive but also have offensive capability — hence their destabilizing danger.

First, Russia has installed a kind of weapon that it's been very good lately at developing: area-denial weapons, such as anti-air missiles, that give Russia the ability to shut down an entire region and prevent NATO from moving in. These are indicated on the map with blue-line circles around Kaliningrad.

Second, Russia has conducted exercises near the Baltics that look at least potentially like they're designed to, if necessary, open a ground corridor from mainland Russia to Kaliningrad. This includes, for example, Russian military flights across or into Baltic airspace, which appear meant to test NATO response times.

The idea would be to prevent NATO from overrunning or isolating Kaliningrad by opening basically a giant military highway to it. But that would mean cutting through the Baltic states that separate them. In other words, it would mean invading them.

The Baltics problem

The dynamic here is that even if Russia's agenda here is purely a worst-case-scenario defensive plan to protect Kaliningrad, it also looks exactly like a plan to invade and seize the Baltic states. As NATO sees Russia building up around the Baltics, it is doing the same.

It's not that US and European leaders think Moscow is going to just up and occupy Latvia out of the blue. Rather, they are in the same situation as Russia is with Kaliningrad: The Baltic states are insecure in ways that require NATO to build them up, and this looks offensive to Russia.

The Baltic states are physically isolated from the rest of Europe. Baltic militaries are very weak compared with Russia's much larger force. And Baltic leaders are convinced, not without reason, that Moscow has designs to launch some sort of Ukraine-style hybrid quasi-war against them — not an all-out invasion, but some sort of potentially violent meddling.

This is why the US has been conducting military exercises in the Baltics and part of why it is quadrupling military investment in Europe: to build up the Baltics as a deterrent against Russia. But the effect of this buildup is to further isolate Kaliningrad, potentially increasing Moscow's paranoia and helping to motivate its own buildup, and so on.

The Eastern Europe security dilemma and the potential for war

These Baltics dynamics are, taken together, a classic example of what political scientists call a security dilemma, in which each side feels insecure and builds up to reach parity, which prompts the other side to do the same.

Because neither side can know the other's intentions for sure, defensive measures are seen as at least potentially offensive, and buildups lead to buildups, which can lead to war.

This is especially dangerous in Eastern Europe because both sides are developing not just stronger but faster military measures, such as air-launched cruise missiles, meant to fight and win any conflict as quickly as possible. This drastically reduces response time, meaning that in case of some provocation or accident that could be misread as something bigger, both sides could have only minutes to decide whether to escalate or deescalate.

The scenarios that could lead to war are discussed in greater depth here. But an accident or misstep is not impossible, given that Russian military jets are already flying in or near NATO airspace with some regularity.

And, yes, this is made all the more dangerous by the presence of nuclear weapons — particularly given Russia's development of small-scale "battlefield" or "tactical" nuclear weapons, and its nuclear doctrine that sets a lower bar for launching warheads than does America's.

To be clear, it is not remotely the case that war is likely. Russian and NATO leaders all want very badly to avoid war, and this is by far the most determinative factor in whether war happens. But this map helps to illustrate how this possibility, while remote, is not unthinkable, either. That, after all, is why the buildup is happening to begin with.