EXPLAINER: What Is Article 5 and How Does it Shape NATO’s Ukraine Response?

Experts say a variety of circumstances at play in Russia’s invasion of Ukraine could lead NATO members to invoke the ‘collective defense’ principle at the heart of their treaty.

Russia’s invasion of Ukraine has faced significant opposition, prompting relentless missile attacks in recent days that have left hundreds dead while Russian troops have circled major cities and spurred what the U.N. high commissioner called the “fastest growing refugee crisis in Europe since World War II.”

NATO member countries have taken steps to thwart Moscow’s advance or otherwise offer Kyiv support, from providing Ukrainian forces with weapons to offering humanitarian assistance to imposing sanctions on Russia and Vladimir Putin himself.

But the alliance is limited in its options. Despite talk of war crimes and human rights violations in Ukraine, NATO members have signaled they are not willing to get directly involved in – or even incidentally drawn into – the conflict by, for example, patrolling Ukraine’s airspace or putting troops on Ukrainian soil.
That’s because of a stipulation in the charter governing the military alliance between 28 European countries and two North American countries dedicated to preserving peace and security in the North Atlantic area. It is routinely referred to as Article 5.

Article 5 specifies that any attack on any of the member countries is effectively an attack against them all. Through the Cold War, when the alliance sought to contain the expansion of the Soviet Union, it served as a deterrent. But in the current conflict, its violation is a guarantee of escalation on a nightmarish scale. Even NATO Secretary-General Jens Stoltenberg has said that NATO allies have a responsibility “to prevent this war from escalating beyond Ukraine.”

“The alliance doesn’t want to invoke Article 5 because it doesn’t want to have a war with Russia,” says Stanley Sloan, a visiting scholar in political science at Vermont’s Middlebury College and author of Defense of the West: NATO, the European Union and the Transatlantic Bargain.

“The best protection against having to do that is Ukrainian success,” Sloan says. “So that’s one reason why the United States and the allies are trying to do everything possible without getting directly involved in the conflict that they can do to help the Ukrainians succeed.”

What is Article 5 and How Does it Operate?

The alliance calls Article 5 the “principle of collective defense at the very heart of NATO’s founding treaty.” Since it was signed in 1949, the alliance has been so powerful and so effective that Article 5 has only needed to be invoked once – after the 9/11 attacks on the U.S.

The charter stipulates that member countries will assist the attacked party “by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.”

Importantly, each nation gets to determine how they will respond, Sloan says. It’s not an automatic commitment to do anything specific in terms of an attack.

After a country is attacked, they would come together with the other alliance members and see if all 30 member countries would agree to regard the situation as an Article 5 case. That human element ensures further protection that an accident or a mistake doesn’t result in a wider war.

The process could be short-lived or could take its time. After the 9/11 attacks on the U.S., member nations took nearly a month to agree to invoke Article 5. But Sloan notes that “active hostilities” in this case would likely make the process much quicker.

Which Countries Are Part of NATO?

The 12 founding members of the alliance were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the U.K. and the U.S. Since its founding in 1949, Greece, Turkey, Germany, Spain, the Czech Republic, Hungary, Poland, Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, Slovenia, Albania, Croatia, Montenegro and North Macedonia have joined. The alliance spread eastward when its principal rival, the Soviet Union, collapsed in 1991 and its former satellite states emerged as fledgling democracies. Putin himself publicly toyed with the idea of Russia joining NATO in the early months of his presidency in 2000.

Ukraine is notably not a member of NATO – an issue that has stood at the center of the increasing tensions from Russia as Ukraine’s ambitions to align itself more with Western countries, including its publicly stated interest in joining NATO, have been met with aggression from Russia.

In fact, a central demand of Russia is to prevent Ukraine from ever joining the military union. The former Soviet state is one of just a few countries in Eastern Europe that aren’t members of the alliance.

And although NATO members are friendly with Ukraine and have condemned Russia’s invasion both verbally and by taking steps to isolate Russia from the global economy, Ukraine’s absence from NATO is key to the alliance’s response. With that distinction in mind, NATO members are walking a fine line.

“NATO is a defensive alliance – our core task is to keep our 30 nations safe,” Stoltenberg said. “We are not part of this conflict and we have a responsibility to make sure it does not escalate and spread beyond Ukraine because that would be even more devastating and more dangerous with even more human suffering. NATO is not seeking a war with Russia.” 

U.S. Secretary of State Antony Blinken has echoed Stoltenberg’s message, saying the alliance would defend “every inch” of NATO territory.

“Ours is a defensive alliance,” Blinken said. “We seek no conflict. But if conflict comes to us, we are ready.”

The response wasn’t always assured. Putin, whose relationship with the Western alliance again turned adversarial as his presidency took on an authoritarian tone, has regularly looked for ways to test the alliance, hoping to divide members with a goal of weakening or dissolving it. Former President Donald Trump reportedly considered withdrawing from the alliance in what would have been a crippling blow to organization amid his strained relations with America’s staunchest allies.

President Joe Biden has forcefully pledged his absolute support to NATO and in an address to the country on the first day of Russia’s invasion reiterated that the U.S. would not engage in war with Russia but said it would meet its commitments – outlined in Article 5 – to defend its NATO partners.

“As I made crystal clear, the United States will defend every inch of NATO territory with the full force of American power,” Biden said. “And the good news is, NATO is more united and more determined than ever. There is no doubt, no doubt, that the United States and every NATO ally will meet Article 5 commitments, which says an attack on one is an attack on all.”

What Does Article 5 Have to do With a No-Fly Zone Over Ukraine?

Among the ways that NATO members have hesitated to get directly involved in the crisis in Ukraine is by establishing a no-fly zone over the country.

Despite repeated calls by Ukrainian President Volodymyr Zeleskyy to do so, a no-fly zone over Ukraine – meaning an area where certain aircrafts would not be permitted so as to prevent airstrikes – would require more involvement than NATO members may be comfortable with.

If no-fly zones were enforced by a NATO member country, some argue that the conflict could broaden beyond Ukraine as a result of Russian and alliance member aircrafts engaging in one way or another.

Conflict between a Russian aircraft and alliance aircraft wouldn’t automatically invoke Article 5, Sloan says, “but it would bring things to the edge.”

“It might be less important whether Article 5 had been invoked at that stage or just the simple fact that the dynamic of the situation is producing a broader conflict,” Sloan says.

And escalating the conflict between Russia and any NATO country would nearly immediately turn the present conflict into a full-blown global crisis.

Stoltenberg said on Friday that “allies agree that we should not have NATO planes operating over Ukrainian airspace or NATO troops on Ukrainian territory.”

“The only way to implement a no-fly zone is to send NATO planes – fighter planes – into Ukrainian airspace, and then impose that no-fly zone by shooting down Russian planes,” Stoltenberg said. “If we did that, we’ll end up with something that could end in a full-fledged war in Europe involving many more countries and causing much more human suffering.”

Could a Cyberattack Invoke Article 5?

Since Russia invaded Ukraine, global alertness to cyberattacks has been heightened, leading U.S. officials to urge American banks and businesses to prepare for attacks in retaliation for imposing severe sanctions on Russia.

Analysts have even been surprised by the lack of serious cyberattacks, shutting off power, inhibiting communications or otherwise impacting Ukraine’s industries after Russia has for years tested its capabilities to mount such attacks. Some have speculated that Russian hackers were surprised or otherwise underprepared for the Russian invasion. Still, those attacks could be imminent.

Senate Intelligence Committee Chairman Sen. Mark Warner warned last week of the Russian hackers’ potential.

“I’m relieved that Russia’s cyberattacks have been fairly limited thus far, but it is clear they have the capacity to do much more, and that it could be potentially devastating for neighboring NATO allies,” the Virginia Democrat wrote in a tweet. “I’m monitoring this closely and getting continuous briefings.”

As the invasion began, Stoltenberg said he believes a cyberattack would also trigger Article 5, reiterating NATO’s earlier-stated position that the alliance will “defend against any aggression toward allies,” whether physical or virtual. But the fear remains that a series of cyberattacks could prompt an asymmetric response in which the latter escalates into the former.

According to Sloan, cyberattacks are a “gray area” when it comes to Article 5, depending on how serious the attack is, with considerations for whether it has a direct impact on people or NATO facilities.

Nevertheless, Biden said when the invasion began that the U.S. is prepared to respond if the cyberattacks become a reality.

“If Russia pursues cyberattacks against our companies, our critical infrastructure, we’re prepared to respond,” Biden said, adding that the government has been working closely with the private sector “to harden their cyber defenses” and “sharpen our ability to respond to Russian cyberattacks.”

What Else Could Trigger Article 5?

Both an intentional attack by the Russians and an unintentional attack on a NATO member country could trigger Article 5.

In terms of intentional attacks, some analysts have had their eyes on the Baltics – countries that don’t border Ukraine but are also former Soviet territory that the Russians could attempt to take back under their control.

The alliance’s eastern flank in general is thought to be at the front lines of any conflict with Russia, leading NATO countries to prioritize strengthening those alliance members in particular.

Accordingly, Biden has reiterated the U.S. commitment to defending its NATO allies, including the eastern flank, by “authorized deployment of ground and air forces already stationed

in Europe to NATO’s eastern flank allies – Estonia, Latvia, Lithuania, Poland and Romania.”

And while few people suspect Putin has designs beyond Ukraine at this point, those countries appear to recognize the threat that Russia’s latest moves pose. Since the Russian invasion began, they’ve been among a handful that have activated Article 4 – another aspect of the treaty that allows member countries to bring security concerns to the alliance for deliberation, which could serve as a prelude to Article 5.

Another intentional attack could involve a Russian challenging Turkey, which last week closed two key passageways – the Bosporus and the Dardanelles – to warships after pressure from the Ukrainian government to do so.

Experts have also identified missile strikes on NATO countries – whether intentional or not – as moves that could trigger Article 5, as Russia has launched hundreds of missiles at Ukraine since the invasion began.

Other unintentional attacks could include spillover from the Ukrainian invasion.

Analysts have been watching the Ukraine’s border with Poland in the west in particular, where Ukrainian forces perhaps retreating into Poland could be fired upon by Russians, potentially leading to an invocation of Article 5. But that possibility is less likely if conflict continues within central and eastern Ukraine.

But more scenarios that could trigger Article 5 are possible.

“This is part of the fog of war,” Sloan says. “You never know once war starts what actually could happen.”


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