WASHINGTON —  President Joe Biden landed in Brussels on Wednesday for urgent meetings this week with members of NATO, the G-7 and the European Union as the continent reels from Russia’s unprovoked war against Ukraine that’s shattered 70 years of relative peace and security in the region. 

As the Kremlin wages its medieval siege war inside Ukraine, just outside the border, more than 35 countries have come together to help tip the scales in favor of Kyiv — the largest voluntary coalition in the history of modern warfare. Missiles, helicopters, Humvees, ammunition, body armor, intelligence reports, money and humanitarian aid are all flowing into Ukraine, where they are having a tangible impact on the course of the conflict.

Thursday’s meetings in Brussels will bring together the world’s most powerful military alliance for an “extraordinary summit” where leaders will decide on troops, sanctions and other measures designed to aid war-torn Ukraine and to bring Russian President Vladimir Putin to his knees.

The next phase of the war

Three pressing threats loom large over the summit, requiring the alliance to figure out its response and whether military intervention would be needed: mistaken fire on an allied nation, cyber attacks to critical infrastructure of a NATO member state and the possibility of chemical or biological warfare within Ukraine, according to experts. 

NATO leaders are also expected to announce more humanitarian aid to Ukraine, particularly the embattled port city of Mariupol, a fresh round of sanctions and new pressure on Moscow’s energy sector.

As the war nears its second month and Russia’s battle deaths soar past 7,000 with almost nothing to show for them, experts say it’s becoming inevitable that Moscow will try new ways to hit back at Kyiv and its backers — both within Ukraine and beyond its frontiers.

Inside Ukraine, the possibility that a desperate Putin could resort to weapons of mass destruction is one of the things that keeps security experts up at night. So does the prospect of a deadlier repeat of last month’s indiscriminate Russian attack on the nuclear reactors at Chornobyl and Zaporizhzhia, where soldiers fired at a reactor that was thankfully offline. 

If Russia had fired on one of the reactors that happened to be online, “that would have caused a nuclear disaster, and we’d basically be looking at trying to evacuate a quarter of Europe — maybe a half of Europe — depending on the wind,” said Scheherazade Rehman, director of the European Union Research Center at George Washington University.

While international nuclear regulators have said the plants are stable and aren’t leaking radiation, the prospect of renewed fighting near them has scientists and European leaders on edge.

Weapons of mass destruction

Frustrated with his slow progress in Ukraine, Putin has been increasingly signaling the possibility of using chemical or biological weapons to wipe out entire cities and force the country to surrender.

On Monday, Biden warned that Putin was falsely accusing the U.S. or Ukraine of using biological or chemical weapons to possibly justify Russia’s own attack on Ukraine.

“They are also suggesting that Ukraine has biological and chemical weapons in Ukraine. That’s a clear sign he’s considering using both of those,” Biden said, without presenting any evidence.

Evidence or not, “the threat of Russia’s use of chemical weapons is real,” said Dan Baer, acting director of the Europe program at the Carnegie Endowment for International Peace.

“Russia has a long track record of accusing others of what they are either already doing or about to do, and that is the kind of projection that we’ve seen in the last couple of weeks. And it’s very scary,” he said in an interview Tuesday. 

Russia has previously used chemical weapons on the battlefield, including in Syria, raising the immediate risk of a chemical attack to Ukraine far higher than it would be if it were any other country that was attacking Kyiv.

“Russia crossing the threshold to the use of chemical weapons for an attack is a greater threat than it was two weeks ago, partly because of Ukraine’s success in defending itself,” said Steven Durlauf, a sociologist at the University of Chicago and an expert in human macroeconomics. 

Russia’s use of chemical weapons would likely overcome any lingering resistance among both the European and American public to support an even greater involvement in the Ukraine conflict, he said. 

On Wednesday, NATO Secretary General Jens Stoltenberg said using chemical weapons would change the nature of the ongoing conflict in Ukraine.

“It will be a blatant violation of international law and with far-reaching consequences,” Stoltenberg said in Brussels, adding that the use of such weapons could impact nearby NATO member countries.

Cyber attacks

The threat of a cyber attack against the United States has evolved so significantly in the past week that the White House has sprung into action, both behind the scenes and in public, to place potential targets of a Russian attack on “shields up” high alert. 

The worry is that Kremlin will launch a major cyber attack on America’s critical infrastructure, likely either against an energy company or a utility provider, Biden administration officials have said.

While there is no doubt the United States would impose grave costs on Russia for any cyber attack that targeted infrastructure Americans rely upon, it’s not a given that the entire NATO alliance would respond exactly the same way if there were a Russian cyber attack against an American, non-military target. 

According to NATO’s founding charter, a cyber attack against one ally is considered an attack against all the allies. Enshrined in the charter’s fifth article, the concept that all NATO members will collectively defend any one NATO member is often referred to simply as the “Article 5” commitment.

“The most important item for NATO to decide with cyber this week is where the line is that triggers an Article 5 response, because it’s all fuzzy right now,” said Rehman, of George Washington University. “Who has to get hurt? And how badly, for us to say, ‘Okay, it’s time’?” 

Smaller nations, like say, Estonia, they may have a harder time convincing members to execute Article 5, Rehman said in a subtle reference to the massive 2007 Russian cyberattack against Estonia that lasted for months and led to a deadly riot.

“But when the Americans shout ‘Article Five,’ no one questions that, and the same applies to any of the larger economies like Germany or France. If they decide to call it, that it crossed the line, then that’s it,” said Rehman. “But then, realize that now we have a third issue: Where that line is, and what it is, and now, for which country?”

The ‘mistake’

There is also a real possibility Russian soldiers could accidentally fire over the border into a NATO member state with Poland identified as the most likely landing spot. 

“The number one item on the table at NATO should be, ‘How do you handle a mistake?'” said Rehman of George Washington University.

“The Russian army is young and inexperienced and they’re sitting on these frozen borders, not knowing what they’re doing or where they’re going, with their communications down,” Rehman told CNBC in an interview Tuesday.

“And now they’ve got [Russian mercenaries] from Mali and Libya in there, who have no clue who the Russians are and who the Ukrainians are, and so they just end up fighting anybody,” she added. A top U.S. commander in Africa recently said the Kremlin is recruiting paid soldiers fighting in African conflicts to come and fight in Ukraine. 

“All it has to be is one shot across the border,” Rehman said. 

More American troops

During his trip to Brussels this week, Biden and NATO allies are expected to commit more troops closer to Ukraine, and to discuss whether to keep them there semi-permanently. 

The United States currently has around 100,000 troops stationed across Europe, with more than 38,000 of them in Germany, according to the Defense Department. But Russia’s invasion of Ukraine has prompted Biden to shift several thousand American soldiers to countries on NATO’s eastern flank. 

Stoltenberg said NATO leaders “would discuss adding four new tactical battalions in Bulgaria, Hungary, Romania and Slovakia which would bring the number to eight battalions to reinforce the eastern flank by Ukraine from the Baltic to the Black Sea.” 

More troops on NATO’s eastern flank is something for which Stoltenberg has long been pushing, and on Wednesday he made it sound like a done deal, saying he expected leaders to agree to “strengthen NATO’s posture in all domains, with major increases in the eastern part of the alliance on land, in the air and at sea.”

Any announcements of an increased American troop presence in Eastern Europe is likely to be warmly received in Brussels this week, but not for the reason one might imagine.

“The reason for Biden to supply an additional, even as many as 10,000 troops, is not that it means they would defend the country’s border from a Russian attack,” said Durlauf. “What it means is that so many Americans will be killed if their countries were attacked that the U.S. couldn’t step aside.”

Durlauf noted that the troop levels under discussion would never be sufficient “to defeat the Russian army in a fight. But they are sufficient to make it impossible for the United States not to fully intervene.” 

In that sense, he said, “the U.S. is giving the Baltics the ultimate establishment credibility, by sending her sons there.”

Humanitarian aid

As the growing refugee crisis strains nations bordering Ukraine to the breaking point, Biden is arriving in Europe prepared to ramp up the U.S. response to the humanitarian crisis.

The U.N. estimated that as of March 23, more than 3.6 million refugees had fled Ukraine to seek refuge in a neighboring country. To put that number in context, that’s roughly equal to the number of Syrian refugees who sought asylum in Europe during the entire first four years of that country’s horrific civil war. 

White House National Security Advisor Jake Sullivan said Tuesday that Biden “will announce further American contributions to a coordinated humanitarian response to ease the suffering of the civilians inside Ukraine and to respond to the growing flow of refugees.” 

The White House declined to say what those contributions would be. But NBC News reported late Tuesday that Biden will announce new plans to offer vulnerable Ukrainians expedited entry to the United States. 

NATO aid to Mariupol

Since the start of the war, the besieged city on Ukraine’s eastern edge has emerged as a symbol to the world of just how much suffering  Putin is willing to inflict upon civilians — and the stoic bravery of average Ukrainians. 

As of Wednesday, 100,000 residents remained trapped in the city without food, clean water, heat, electricity or medical supplies, amid relentless Russian bombardment. 

“What’s happening now in Mariupol is a massive war crime, destroying everything, bombarding and killing everybody,” Josep Borrell, the European Union’s foreign policy chief said Monday in Brussels.

As NATO leaders descend on Brussels ahead of Thursday’s meeting, the question of what will happen to Mariupol is beginning to look like the first major test of how far Putin, and NATO, are willing to go.

One early indication came on Wednesday, while Biden was still in the air on his way to Europe. Secretary of State Antony Blinken formally announced that the United States government believes Russia has committed war crimes in Ukraine and should be prosecuted.

In a statement, Blinken repeatedly raised the brutality in Mariupol, and he compared it to similar Russian campaigns against Grozny in the Second Chechen War and Aleppo during the Syrian civil war.

Despite having pledged not to send troops into Ukraine, some experts believe there is much more NATO can do for Mariupol.

“NATO can conceivably coordinate a relief mission to Mariupol,” said Matt Schimdt, a professor of national security and political science at the University of New Haven.  “NATO could also set up a humanitarian safe zone that isn’t a [no-fly zone] — a ground-up system that uses anti-missile and anti-artillery technology.” 

“These options push the red lines Putin has established,” said Schmidt. “But NATO must get creative and find a way to break Putin’s ability to hold populations hostage.”

A broader energy embargo

There are growing signs this week that Europe and the United States are prepared to go farther than ever before towards an embargo on Russian oil and gas, but it was unclear Wednesday what would be announced while Biden is in Brussels. 

“The most important step in terms of sanctions is also the most complicated, and that is that the West needs to cut off all purchases of oil and energy supplies from Russia,” said Durlauf, of the University of Chicago.

The United States already banned imports of Russian oil and gas in early March, a decision made much easier by the fact that America is a producer of oil and gas. Few European countries can say the same.

On Monday, EU foreign ministers reached an impasse over a full embargo on Russian oil, with Germany reportedly leading the bloc of hesitant countries. 

New sanctions on Russian elites 

Biden is also meeting in Brussels with leaders of the EU and the G-7 group of developed economies, which plan to announce fresh sanctions on Russian elites and members of the government. 

“The sanctions have been extraordinarily rapid and extraordinarily powerful, but the next steps have to be taken to continue to put pressure on Russia — be it the elites, be it the regime itself or be it the population,” said Durlauf.

Specifically, the joint alliances will unveil a package of new individual sanctions targeting approximately 400 people, the Wall Street Journal reported on Tuesday. This will include more than 300 lawmakers from the Russian lower house of parliament, the Duma and Russian elites.

“In an authoritarian regime like this, it’s complicated to know where we exert pressure. But the sanctions put pressure on all the dimensions of the polity, and that has to be continued and has to be enhanced,” Durlauf told CNBC.

End game?

Despite Russia’s overwhelming military advantage, few American or European experts anticipate that the war will be over any time soon. 

Military experts almost universally agree that the biggest factor slowing Russia’s advance is not Western assistance, but rather Ukraine’s own fierce resistance.

“So what’s the capacity of the Ukrainian people to fight and hold out? I’m not sure there is any limit to it,” said Durlauf, of Chicago.

“What we’re seeing is the birth of a nation,” he said. “Ukraine has existed as a separate country for about 30 years, but now the national myths that are going to define them for the next 200 years are being written.”

— CNBC’s Amanda Macias contributed reporting to this story.