A malfunctioning missile stopped a Russian jet from shooting down a UK spy plane. Here's what could have happened if it had succeeded.

• A hair-raising encounter saw a Russian jet almost shoot down a British spy plane last year.
• Incidents like this raise fears of NATO's Article 5 being triggered, leading to an all-out war.

A hair-raising encounter between Russian and British military aircraft, which resurfaced in the news last week, raised the question of near-misses and whether a trigger-happy incident between global rivals could spill over into World War III.

On September 29 last year, a Russia Su-27 fighter jet fired on a UK surveillance plane in international airspace over the Black Sea, near Russian-occupied Crimea.

The British plane was unharmed, and the Kremlin assured the British Minister of Defense that there had been a "technical malfunction."

But it later emerged, per two anonymous intelligence sources who spoke to The New York Times, that the Russian pilot had misheard ground communications, and believed he had permission to fire.

Only a malfunction stopped the missile smashing into the plane.

Leaked Pentagon documents, circulated this month, also referred to the incident as a "near shoot-down."

The new reporting painted a much more serious picture of the incident, raising questions of what the international response would have been had it succeeded.

As a member of NATO, the UK adheres to the principle of collective defense, whereby — through Article 5 — an attack on one member state can be interpreted as an attack on all.

Insider spoke to a former NATO officer and expert on international security to find out how incidents of this kind are handled at the highest level.

Diplomatic backchannels

It's often imagined that activating NATO Article 5 is an automatic process, with events ever-ready to spiral out of control.

In reality, it's a voluntary and vanishingly rare step that — even outside of the desire to avoid a massive war — countries have political incentives not to take, William Alberque, director of strategy, technology and arms control at the International Institute for Strategic Studies, told Insider.

In fact, NATO countries generally want to demonstrate that they can defend themselves without having to turn to other member states for help, he said.

There's also an unspoken risk — if no other nation agreed to defend them "that would be really damaging," he added.

According to Alberque, the only time Article 5 was triggered was September 11, 2001, and even then the US was reluctant to use it because it didn't want it to be a NATO issue.

"They wanted US unilateral action to rule the day," he said.

In fact, throughout the Cold War there were instances of planes being shot down on both sides, and despite all this it never resulted in Article 5 being triggered.

And more recently, when a stray Ukrainian missile landed in Poland last year, killing two Poles and setting off an initial blame game between Russia and Ukraine, Poland also didn't want to invoke Article 5.

"Turns out they didn't even want Article 4. They just went in for a regular NATO consultation because they didn't think that it rose to the level of acute," Alberque said.

Article 4, which results in a consultation between all member states on what to do next, can be invoked when a country believes its security is threatened. It's been used seven times in NATO's history.

Russia's risky provocations

At the same time, Russia regularly needles its rivals by tailing their aircraft when they approach areas it considers sensitive, according to Alberque, who is also a former director of NATO's arms control unit.

"This is actually a political signal from Russia that they don't want you to be there," he said.

Alberque said this isn't unique to Russia – many countries conduct air intercepts. But "the problem with the Russians is they often do stupid stuff."

"When you fly near Crimea, when you fly near Kaliningrad, when you fly near St. Petersburg, they try to send aircraft out to intimidate you into backing off."

He added that this is often done on the instruction of the Kremlin, which has become more aggressive in its approach since 2013.

Alberque recalled an infamous encounter over the Baltic Sea in 2016, when armed Russian Su-24 fighter jets harassed the USS Donald Cook in international waters, coming within 30 feet of the ship at extremely low altitude.

An alternative to NATO's Article 5

Regarding the incident last year, even if a British pilot had died, the UK would likely have taken a long pause before rushing to invoke Article 5, Alberque says.

"There's going to be all kinds of reactions, there's going to be a debate in parliament," he says. "People will try to make political hay out of it, but wise leaders will say: 'No, this wasn't world war. Let's calm down here.'"

Meanwhile, the UK and Russia — like the US – are also signatories to a treaty known as INCSEA.

First ratified during the Cold War, it sets out a number of rules designed to prevent accidents in international waters and airspace.

In provocative situations like this, it offers a less aggressive channel, where UK authorities could have reminded Moscow of its right to fly over international waters and of the risk of triggering a war.

Or, in Alberque's words, rather than Article 5, the UK could have used INCSEA to consult with Russia and simply say: "'What the fuck happened here?'"