Biden increases NATO troop presence and imposes sanctions as concerns of Russian chemical attacks escalate.

• The summit faces three significant challenges, which demand the alliance to determine its course of action and whether military intervention is necessary.
• The possibility of a mistaken fire on an allied nation, cyber attacks on a NATO member state, and the possibility of chemical or biological warfare within Ukraine pose a threat to global security.
• NATO leaders are anticipated to declare additional humanitarian aid to Ukraine, specifically focusing on the beleaguered port city of Mariupol, as well as announce a new set of sanctions and intensify pressure on Moscow's energy sector.

President Biden arrived in Brussels on Wednesday for crucial talks with NATO, G-7, and EU leaders as the continent grapples with the aftermath of Russia's unprovoked war against Ukraine, which has disrupted 70 years of peace and stability in the region.

The Kremlin is waging a medieval siege war inside Ukraine, but more than 35 countries have come together to aid Kyiv, making it the largest voluntary coalition in modern warfare. Missiles, helicopters, Humvees, ammunition, body armor, intelligence reports, money, and humanitarian aid are all being sent to Ukraine, where they are making a tangible impact on the conflict.

An "extraordinary summit" is scheduled for Thursday in Brussels, where leaders of the world's most powerful military alliance will gather to discuss and decide on measures to aid war-torn Ukraine and put Russian President Vladimir Putin under pressure.

The next phase of the war

The alliance must decide whether military intervention is necessary to address three imminent threats: a mistaken fire on an allied nation, cyber attacks on a NATO member state's critical infrastructure, and the possibility of chemical or biological warfare in Ukraine, as experts predict.

NATO leaders are anticipated to declare additional humanitarian aid to Ukraine, specifically focusing on the beleaguered port city of Mariupol, as well as announce a new set of sanctions and intensify pressure on Moscow's energy sector.

With Russia's battle deaths surpassing 7,000 after two months of war and little to show for their efforts, experts predict that Moscow will likely attempt new strategies to retaliate against Kyiv and its supporters, both within Ukraine and beyond its borders.

The possibility of a desperate Putin using weapons of mass destruction and 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, keep security experts up at night inside Ukraine.

If Russia had targeted one of the active reactors, it would have resulted in a nuclear disaster, necessitating the evacuation of a significant portion of Europe, possibly half of it, based on wind direction, according to Scheherazade Rehman, director of the European Union Research Center at George Washington University.

The possibility of renewed conflict near international nuclear plants has left scientists and European leaders concerned, despite regulators stating the plants are stable and not leaking radiation.

Weapons of mass destruction

Putin has been increasingly hinting at the possibility of using chemical or biological weapons to eliminate entire cities in Ukraine and force the country to surrender due to his frustration with slow progress.

On Monday, Biden stated 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.

Biden stated that the suggestion that Ukraine possesses biological and chemical weapons is a clear indication that he is considering using them, despite not providing any evidence.

According to Dan Baer, the acting director of the Europe program at the Carnegie Endowment for International Peace, the threat of Russia's use of chemical weapons is real.

In an interview on Tuesday, he stated that Russia's history of accusing others of actions they are already doing or about to do is a form of projection, which is very concerning.

The use of chemical weapons by Russia in Syria increases the likelihood of a chemical attack on Ukraine, making it a higher risk than if any other country were attacking Kyiv.

The threat of Russia using chemical weapons for an attack has increased due to Ukraine's success in defending itself, according to Steven Durlauf, a sociologist at the University of Chicago and an expert in human macroeconomics.

The use of chemical weapons by Russia would likely increase support for greater involvement in the Ukraine conflict among both the European and American public, according to him.

Jens Stoltenberg, NATO Secretary General, stated on Wednesday that the use of chemical weapons would alter the course of the conflict in Ukraine.

Stoltenberg stated in Brussels that the use of such weapons would be a clear violation of international law and could have severe consequences, affecting not only the country in question but also neighboring NATO member states.

Cyber attacks

The White House has taken measures to protect potential targets of a Russian cyber attack against the United States, following a significant increase in the threat in the past week.

Officials from the Biden administration have expressed concern that the Kremlin may launch a significant cyber attack against America's critical infrastructure, possibly targeting an energy company or a utility provider.

It is not certain that all NATO members would react similarly to a Russian cyber attack against an American non-military target, despite the United States imposing significant costs on Russia in response.

As per NATO's founding charter, a cyber attack against one ally is considered an attack against all the allies. This principle, enshrined in the charter's fifth article, is commonly known as the "Article 5" commitment, which obligates all NATO members to collectively defend any one NATO member.

Rehman, of George Washington University, stated that the crucial decision for NATO to make with cyber this week is determining the boundary that triggers an Article 5 response, as it is currently unclear. The question arises: "Who must be harmed, and to what extent, for us to declare, 'It's time'?"

Estonia, being a smaller nation, may face challenges in persuading members to invoke Article 5 in the event of a cyberattack, as alluded to by Rehman in reference to the 2007 Russian cyberattack that lasted for months and resulted in a deadly riot.

When the Americans shout 'Article Five,' no one questions it, and the same applies to larger economies like Germany or France. If they decide to call it a violation, that's it. However, now we have a third issue: determining where that line is and what it means for each country.

The ‘mistake’

It is possible that Russian soldiers could unintentionally shoot across the border into a NATO country, with Poland being the most probable location.

"At NATO, the top priority should be addressing the question, 'How do you handle a mistake?'" according to Rehman of George Washington University.

According to Rehman, the Russian army is inexperienced and sitting on frozen borders, unsure of their actions and direction, with communication systems down.

The Kremlin is reportedly enlisting Russian mercenaries from Mali and Libya to fight in Ukraine, despite their lack of knowledge about the conflict and the Russian and Ukrainian forces involved.

"One shot across the border is all it needs," Rehman stated.

More American troops

Biden and NATO allies are anticipated to increase their troop presence near Ukraine during his Brussels trip this week and discuss the possibility of keeping them there permanently.

Currently, the US has approximately 100,000 troops stationed in Europe, with over 38,000 in Germany. However, due to Russia's invasion of Ukraine, Biden has relocated several thousand American soldiers to countries on NATO's eastern border.

NATO leaders will discuss adding four new tactical battalions in Bulgaria, Hungary, Romania, and Slovakia, increasing the number of battalions to eight, to reinforce the eastern flank by Ukraine from the Baltic to the Black Sea, as stated by Stoltenberg.

Stoltenberg has long advocated for more troops on NATO's eastern flank, and on Wednesday he stated that he anticipated leaders would agree to enhance NATO's posture in all domains, with significant increases in the eastern part of the alliance on land, in the air, and at sea.

The announcement of an increased American troop presence in Eastern Europe is likely to be met with approval in Brussels this week, not because of the security benefits it brings, but due to the economic benefits it brings to the region.

According to Durlauf, the purpose of Biden providing an additional 10,000 troops is not to defend the country's border from a Russian attack. Instead, it means that so many Americans would be killed if their countries were attacked that the U.S. couldn't remain neutral.

Durlauf pointed out that the troop levels being discussed would not be enough to defeat the Russian army in a fight. However, they are enough to ensure that the United States cannot remain neutral.

He stated that the U.S. is granting the Baltics the highest level of establishment recognition by deploying her sons there.

Humanitarian aid

Biden is heading to Europe to intensify the U.S. reaction to the escalating refugee emergency.

As of March 23, the U.N. estimated that over 3.6 million refugees had fled Ukraine to seek refuge in neighboring countries. This number is equivalent to the total number of Syrian refugees who sought asylum in Europe during the first four years of that country's civil war.

On Tuesday, Jake Sullivan, the White House National Security Advisor, stated that Biden plans to reveal additional American aid to assist the humanitarian effort in Ukraine and address the escalating refugee crisis.

The White House refused to reveal the nature of the contributions. However, NBC News reported on Tuesday evening that Biden will unveil new plans to facilitate quicker access to the US for vulnerable Ukrainians.

NATO aid to Mariupol

The city on Ukraine's eastern edge, which has been under siege since the start of the war, has become a symbol to the world of the immense suffering Putin is willing to inflict on civilians and the remarkable bravery of ordinary Ukrainians.

Since Wednesday, 100,000 people have been stranded in the city without access to food, clean water, heat, electricity, or medical supplies due to continuous Russian attacks.

Josep Borrell, the European Union's foreign policy chief, stated on Monday in Brussels that the current situation in Mariupol constitutes a massive war crime, resulting in the destruction of everything, bombardment, and the killing of everyone.

The upcoming NATO meeting in Brussels will serve as the first major test of Putin's and NATO's willingness to go.

On Wednesday, while Biden was en route to Europe, Secretary of State Antony Blinken declared that the US government believes Russia has committed war crimes in Ukraine and should be held accountable.

Blinken frequently mentioned the brutality in Mariupol and compared it to Russian campaigns against Grozny in the Second Chechen War and Aleppo during the Syrian civil war.

Although NATO has vowed not to deploy troops in Ukraine, some analysts suggest that the alliance could take further actions to aid Mariupol.

"NATO can coordinate a relief mission to Mariupol and set up a humanitarian safe zone with anti-missile and anti-artillery technology," said Matt Schimdt, a professor of national security and political science at the University of New Haven.

Schmidt stated that the options presented by these individuals exceed the boundaries that Putin has set, but NATO must devise a creative solution to disrupt Putin's ability to control populations.

A broader energy embargo

It is uncertain what will be announced regarding an embargo on Russian oil and gas while Biden is in Brussels, despite indications that Europe and the United States may take unprecedented steps towards it.

Cutting off all purchases of oil and energy supplies from Russia is the most complicated step in imposing sanctions, according to Durlauf of the University of Chicago.

While the US has prohibited Russian oil and gas imports since early March, many European countries lack the ability to do so due to their dependence on these resources.

On Monday, EU foreign ministers failed to agree on a full embargo on Russian oil, with Germany reportedly leading the group of countries that opposed the measure.

New sanctions on Russian elites

Biden is meeting with EU and G-7 leaders in Brussels to announce new sanctions on Russian elites and government members.

Durlauf stated that the sanctions imposed on Russia have been swift and effective, but further actions must be taken to maintain pressure, whether targeting the elites, regime, or population.

On Tuesday, the Wall Street Journal reported that the joint alliances will reveal a set of new individual sanctions affecting approximately 400 individuals, including more than 300 lawmakers from the Russian lower house of parliament, the Duma, and Russian elites.

It is challenging to determine where to apply pressure in an authoritarian regime, but sanctions can put pressure on all aspects of the political system, which must be maintained and intensified, as Durlauf stated to CNBC.

End game?

Although Russia has a significant military advantage, many American and European experts predict that the war will not end soon.

Almost all military experts concur that the primary reason for Russia's slow progress is not external aid, but rather Ukraine's tenacious opposition.

Durlauf, of Chicago, stated that the Ukrainian people's capacity to fight and endure is uncertain, as there may be no limit to it.

The birth of a nation is taking place in Ukraine," he stated. "For the past 30 years, Ukraine has been an independent country, but now the national myths that will shape their identity for the next 200 years are being created.

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