What would happen if Russia invaded another Nato country?

Ukraine is under attack – but Western troops will not rush to its defence.

The US and other allies of the under-siege nation have offered military aid and will hit Russia with a tough package of sanctions.

Fighting Kremlin fire with fire, however, is off the table – unless in the unlikely event an important line on the map is crossed.

Ukraine is not a member of the North Atlantic Treat Organisation (Nato), a defence alliance encompassing the US and most of Western Europe.

Under the terms of the agreement signed between the parties, all members have to come to the defence of a country if it is attacked.

The article five powers have only been invoked once following the September 11, 2011, attacks in New York so it’s unclear how its activation would play out.

Any military response would inevitably be primarily made up of US forces and hardware but as the country with the second largest defence budget, the UK would doubtless play a critical role.

Nato has its own command infrastructure in place but in reality, where defence is concerned, national governments tend to make the weather.

The alliance’s standing forces and integrated missile system would all kick into effect in the event of any attack on its soil.

Some countries which are concerned about Russian activity on their own border are signed up to Nato.

Since German reunification in 1990, there has been a steady slide of countries turning west for defence guarantees away from Russia.

Estonia and Latvia – which border Russia and have called for more support – are members, as are Lithuania and Poland which border Russia’s puppet state, Belarus.

An attack on any of them would warrant an immediate armed response from the UK, US and other European allies.

Neither Western governments or Moscow want to get into a shooting match with another nuclear power, meaning seeing similar scenes to what has happened in Ukraine play out elsewhere in Europe highly unlikely.

British defence secretary Ben Wallace hosted a meeting of the UK-led Joint Expeditionary Force this week, a military alliance with Denmark, Finland, Estonia, Iceland, Latvia, Lithuania, the Netherlands, Sweden and Norway.

It was only formed in 2012 but could be thrust into the spotlight if Finland experiences heightened border tensions with Russia.

The European Union also has some limited collective defence arrangements but these are far less entrenched or tested than Nato structures.

Ukraine’s aspiration to one day join Nato – which Vladimir Putin considers a red line – is one of the underlying causes of the conflict.

Nato is seeking to broaden the resistance to Russia’s actions by engaging with non-members Finland and Sweden, both of which will be represented at an emergency summit tomorrow.

Nato’s general secretary Jen Stoltenberg said today: ‘Peace on our continent has been shattered.

‘Russia is using force to try to rewrite history, and deny Ukraine its free and independent path.’

The activation of Nato’s ‘graduated response plans’ is a rare step that Stoltenberg said underscored the gravity of a ‘full-fledged invasion’ of Ukraine, giving allied commanders greater scope for decision-making.

He added: ‘This is a deliberate, cold-blooded and long-planned invasion.

‘Russia’s unjustified, unprovoked attack on Ukraine is putting countless innocent lives at risk with air and missile attacks.’

He said there were no Nato troops in the country, adding ‘what we do is defensive’.

Downing Street said the UK is increasing its ‘air policing contribution’ to Nato, which will be used to bolster patrols over the borders of Romania and Poland.

They added: ‘This is a defensive capability designed to protect the airspace of our Nato allies … they will not operate in Ukrainian or Russian airspace.’

While most analysts say the prospect of a general war in Europe remains remote for the time being, general Sir Richard Shirreff warned one would begin if Russia puts ‘one bootstep’ on Nato territory.

Asked whether Britain might be expected to take part directly in the current military confrontation, he told the BBC: ‘Absolutely there is a possibility that we as a nation could be at war with Russia, because if Russia puts one bootstep across Nato territory, we are all at war with Russia. Every single one, every single member of the Nato alliance.

‘Article 5 (of the Nato alliance) says an attack on one is an attack on all, so we need to change our mindset fundamentally, and that is why I say our defence starts in the UK on the frontiers of Nato.’

Get in touch with our news team by emailing us at webnews@metro.co.uk.

For more stories like this, check our news page.