If Russian troops cross into Nato territory, does that mean war?

If Russia’s troops cross into Nato territory, does that mean war? World peace has been wagered for 70 years on the assumption that it does. But it doesn’t – not necessarily.

Such an action would trigger Article 5 of the North Atlantic Treaty, the agreement which all Nato member states are party to, which states that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all”. But it doesn’t necessarily follow that the response to an armed attack on one Nato member would be a military one. Article 5 was written in 1949 and has never been triggered by an attack from another country. If Nato was compelled to trigger it as a result of Russian aggression, we’d be in new territory.

Would a Nato response be an armed one? The exact wording of Article 5 states that, in the event of an attack against a Nato state, the alliance will take “such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area”. Note that an armed response from Nato isn’t mandatory – it’s just deemed acceptable in the circumstances. In theory, the necessary action could just be more sanctions.

In all likelihood, a Russian invasion of a Nato country would result in a military response from the alliance. Just a few days before Russia’s invasion began, Joe Biden made it clear that the US would “defend every inch of Nato territory”. But Article 5 is largely untested. It has only been invoked in the wake of the 9/11 attacks – and never to defend a member against another sovereign state.

Would the triggering of Article 5 reveal a hierarchy among the – in theory equal – member states? If so, this would undermine Nato’s entire raison d’etre. But it’s worth asking whether, for instance, an attack on the advanced economies of Britain, France or Germany would trigger a military response from the US, while an attack on one of the Baltic states – less strategically important to America and, being cushioned between Russia, Belarus and the Russian exclave Kaliningrad, hard to defend – would merely prompt a nonviolent response?

“As you can imagine, when you get a couple of dozen Russian cruise missiles hitting a Ukrainian base… just 20km from the Polish frontier, it makes many Nato allies very nervous,” Jamie Shea, the former deputy assistant Nato secretary general, told Tortoise last week. Washington’s response to the Ukraine crisis so far – facilitating the supply of some equipment to Ukraine but not others, such as jets – suggests that the Biden Administration is carefully weighing the amount and type of materiel they’re giving to Ukraine against the risk of escalation into a hot war with Russia.

What would constitute an “attack” on a Nato member state? Shea said that in an age of hybrid warfare, the threshold for a response from Nato was probably still a physical attack. “If you tell somebody like Putin ‘you can attack me up to here, and I’m not going to do anything… but if you go above this line, then I’m going to respond’, you obviously encourage your adversary to engage in those kinds of hybrid war tactics, like cyber, thinking that it will be treated with indifference, or at least impunity.” This is the territory Russia has occupied in its campaign against the West in recent years – proxy wars in Ukraine and Syria; the poisoning of Putin’s enemies on the UK’s streets; the interference in Western elections and referendums – and so far none has been considered an attack in the sense that it warrants a Nato response as outlined under Article 5.

But imagine an attack on a Baltic state conducted by “militias” whom the West suspected to be unmarked Russian soldiers, and who were welcomed by Russian-speaking communities after a barrage of disinformation from the Russian state television they consume. This is what happened in Crimea in 2014, and the leaders of Estonia, Latvia and Lithuania say it could happen in their countries. How decisively would Nato respond? Would the ambiguity of the situation prove convenient for US and western European leaders unwilling to risk nuclear war?

The invasion of Ukraine has given Nato renewed purpose. The branding of the alliance by various Western leaders as “brain-dead” (Macron) or “obsolete” (Trump), has been proven wrong by a predatory, tyrannical Russia once again unleashing terror on eastern Europe. But its response to a direct attack will be its ultimate test.