In recent weeks Moscow has been increasing its military presence in the region, on land and at sea, raising fears of an invasion.

This is what perhaps prompted President Biden to call President Vladimir Putin on Tuesday morning. Biden proposed the two hold a summit “to discuss the full range of issues,” but the situation in Ukraine was certainly on top of the American president’s mind.

Of course, Russia already had a significant military presence there, since it annexed Crimea in 2014 and pro-Russian separatists seized part of eastern Ukraine’s Donetsk and Luhansk regions.

But now, with the West watching with confusion and nervousness, Russia is mobilizing new units to these areas. Videos have circulated on TikTok showing trains moving tanks from faraway Siberia, and of units from the crack 76th Airborne Division in Pskov heading from their base near Estonia for Crimea.

Together it’s the largest concentration of forces near the border with Ukraine since 2014. Clearly, these troops are not on a holiday excursion.

Tensions are rising fast. Not only is the military buildup happening as fighting intensifies at the contact line in the Donetsk region, but there has also been a significant escalation on Russian rhetoric against Ukraine in the past few weeks.

State media in Moscow has promoted absurd accusations that Kiev is planning atrocities, with the implicit message that this has to be stopped. Then a few weeks ago, the leading presenter in the evening news show, Dmitry Kiselyov, talked about a “second fratricidal” war coming in Ukraine, implying that Russia had nothing to do with it all.

So what’s the Kremlin up to?

An outright invasion of Ukraine (as part of an attempt to control southern Ukraine in order to secure the water supply to Crimea) would be the beginning of large-scale war on the plains of Eastern Europe. Ukrainian forces will resist, and fighting might well be extensive and prolonged.

The West must take decisive action. Russia could face severe penalties, such as getting kicked out of the international financial system as part of a response. If that were to happen, not even China might be able to rescue the Russian economy.

There does not seem to be much logic in such a Kremlin course of action. But Moscow could also be repeating the 2014-2015 playbook, when Russia threw significant military resources into defeating the Ukrainian forces, and then sat down for a political negotiation to extract significant concessions.

That’s what happened with the Minsk agreements. Up against the wall, and having suffered severe losses on the battlefield, Ukraine didn’t have much room to negotiate in its favor.

At the moment, relations between Moscow and Washington — and between Brussels and Kiev — are near rock bottom. We’ll see how Putin reacts to Biden’s call, but he certainly did not appreciate Biden saying he was a “killer.” With Brussels, it has been downhill since Foreign Minister Sergei Lavrov’s humiliating treatment of E.U. High Representative Josep Borrell during his Moscow visit. And relations with Kiev seem to have deteriorated even further since President Volodymyr Zelensky took action against the leading Moscow proxy on the domestic political scene.

The Kremlin might think that it can’t get much worse, and that this might be the time to do some “correction” of the situation with Ukraine. Provocation has worked before — perhaps it could work again.

There’s no sure way of knowing where things are headed.

But Russia must face a united and robust diplomatic front. Putin must understand there will be severe consequences if there’s further aggression against Ukraine.

This is a crucial test for the United States and Europe. The security and stability of the continent is on the line.

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