Russia’s repeated violations of NATO airspace in the past few weeks are raising a difficult question for the alliance: Whether to respond by shooting down Russian aircraft.

On Friday, three Russian MiG-31s violated Estonian airspace for 12 minutes before being intercepted by NATO fighter jets, including Italian F-35s participating in the alliance’s enhanced Baltic air policing mission. Earlier this month, Russian drones entered Poland’s territory and at least three were shot down after Polish F-16s and Dutch F-35s responded.

“Russia should be in no doubt: NATO and allies will employ, in accordance with international law, all necessary military and non-military tools to defend ourselves,” NATO ambassadors said in a statement released on Tuesday.

Moscow insists it’s done nothing wrong.

Several NATO countries including Poland and Sweden are now warning they’re ready to shoot down Russian aircraft entering their airspace.

But according to Charly Salonius-Pasternak, CEO of the Helsinki-based Nordic West Office think tank, there is a fundamental difference between Moscow and NATO. “Russia has said they think they’re in a military conflict with us and the West. We do not see it that way and because of that, our rules of engagement are different,” he said.

Here’s what you need to know about NATO’s response so far.

1. What are NATO’s rules of engagement?

The alliance’s rules of engagement are classified.

They define “the parameters for what the military can do within any given situation, that means that they are very different depending on the mission or the operation,” explained Oana Lungescu, a former NATO spokesperson who is currently a distinguished fellow with the Royal United Services Institute think tank in London.

Rules of engagement must be in line with the alliance’s political guidance, she added. “For NATO, the main political guidance is that it’s a defensive alliance, whose aim is to deter aggression and prevent any conflict, and should that fail, to defend against it and defeat it.”

They are approved by the North Atlantic Council (NAC), which gathers all NATO allies. They are implemented by the Supreme Allied Commander Europe (SACEUR), currently Gen. Alexus Grynkewich, a U.S. Air Force general who also commands U.S. forces in Europe. Like every political decision within the alliance, rules of engagement require consensus.

2. What about national armed forces?

NATO rules of engagement do not prevent national armed forces from making their own decisions on their soil when they are under national command.

On Tuesday, Lithuania adopted new rules allowing its military to react to airspace violations “earlier and faster.” Romania, whose airspace was violated multiple times by Russian drones in the past months, on Thursday convenes its Supreme Defense Council to define the rules of engagement in case of more incursions by drones or manned aircraft.

However, there can be risks if a country’s unilateral action leads to escalation, as Polish Prime Minister Donald Tusk hinted on Monday.

“We are ready for any decision aimed at destroying objects that may threaten us, such as Russian fighter jets,” he said during a press conference, while adding that some kind of consensus must also be reached among NATO allies.

“I must be 100 percent certain … that all our allies will treat this in exactly the same way as we do. I must be 100 percent certain that when the conflict enters such an acute phase, we will not be alone in this,” the Polish prime minister continued. “We need to think twice before deciding on actions that could trigger a very acute phase of the conflict.”

But that scenario would not have applied to Friday’s situation over Estonia because Baltic states do not have their own fighter jet fleets and rely on NATO’s air policing missions.

According to a former NATO official, it’s precisely because Estonia doesn’t have its own warplanes that Russia violated the country’s airspace: “If the intrusion had taken place over Finland, the Finns could have decided to take it out. In the Baltic airspace, we are obliged to go up the NATO chain of command.”

3. Why didn’t NATO shoot down the Russian fighter jets?

There are several reasons — both political and military — why NATO’s warplanes didn’t take down the Russian MiG-31s last week.

“Decisions on whether to engage in shooting aircraft … are always based on available intelligence regarding the threat posed by the aircraft,” NATO Secretary-General Mark Rutte said Tuesday, arguing that “no immediate threat” was detected in Estonia.

He echoed Estonia’s Prime Minister Kristen Michal, who said “there are certainly various parameters for the use of force,” hinting that Friday’s incident didn’t call for it despite its obvious intentional nature.

NATO’s response was in line with procedure, Estonia’s former air force chief Jaak Tarien told local media, adding that force is not immediately used during peacetime, under the assumption the incursions could be a mistake.

According to Mykola Bielieskov, a Ukrainian military analyst and research fellow at Ukraine’s National Institute of Strategic Studies, both the risk of escalation as well as uncertainty about Donald Trump’s stance played a role in how NATO reacted.

“Conditionally, no one will start World War III because of this,” he said. European countries “think that NATO is restrained by the fact that there is no certainty about the reaction and position of the U.S. under the Trump administration.”

4. Are NATO countries looking to change the rules of engagement?

It’s unclear whether the current rules of engagement allow NATO fighters to shoot down Russian warplanes — and if so, under what circumstances.

Inside the alliance, the general sense is that NATO’s defensive capabilities are up to the job.

“There’ll be lots of debates” after Estonia, said one NATO diplomat, who was granted anonymity to speak candidly, “but there was no sense we were behind the curve.”

There is an ongoing debate as to whether the rules for NATO’s air policing missions should be tightened. “Discussions will continue in NATO on appropriate measures and responses. It is important that Russia changes behavior,” said a senior NATO diplomat.

Top officials in Estonia, Lithuania and the Czech Republic are calling for a more forceful response the next time Russia tests NATO, namely taking down Moscow’s warplanes. “We must respond appropriately, including possibly shooting down Russian aircraft,” said Czech President Petr Pavel.

Speaking on the sidelines of the U.N. General Assembly on Tuesday, the U.S. president was asked if he believed NATO countries could shoot down intruding Russian aircraft. “Yes, I do,” he responded.

“Roger that,” responded Polish Foreign Minister Radosław Sikorski.

5. How could NATO change the rules of engagement?

In most cases, changing the rules of engagement would have to go through the NAC, where NATO allies would make a consensus-based decision relying on military advice and discussions with SACEUR Grynkewich.

Since the full-scale war in Ukraine started, however, SACEUR has more authority to make adjustments without going through the NAC — such as sending more ships or planes to a given area.

“NATO’s defense plans were activated on Feb. 24, 2022. Those plans … continue to be adjusted, that also gives SACEUR significant authority over how he uses assets and forces,” said Lungescu, the former NATO spokesperson.

6. What happened the last time a NATO country shot down a Russian aircraft? 

In 2015, Turkey shot down a Russian Sukhoi-24M near the Turkish-Syrian border after a 17-second airspace violation and following several warnings.

Crucially, however, that wasn’t part of a NATO mission — unlike Friday’s incident — but rather under national authority. “They had their own air force and took the decision nationally and only informed allies (in detail) afterwards. It proved to be highly effective,” the senior NATO diplomat mentioned above said.

Ankara triggered NATO’s Article 4 and played the warning audio to the NAC, which then issued a statement. After that, the military alliance helped Turkey monitor its airspace with more AWACS early warning aircraft.

According to Estonia’s Tarien, the incident had consequences: Moscow imposed trade sanctions on Ankara, and Turkish President Recep Tayyip Erdoğan ultimately apologized.

However, Russian planes stopped flying through Turkish airspace.

Veronika Melkozerova and Jan Cienski contributed to this report.

This article has been updated.