What is Article 4?

Article 4 of the North Atlantic Treaty stipulates: "The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened."

In this case, it essentially means that when a member is threatened by military action that could harm its territorial claims, the other members are required to consult one another.

Other NATO countries have agreed to the call made by Estonia and other eastern European nations.

However, Ukraine is not a member of NATO - but has been is favour of joining the security alliance for a number of years.

As soon as the article is invoked, the issue is discussed and can potentially lead to some form of joint decision or action on behalf of the alliance.

Fellow members sitting around the Council table are encouraged to react to a situation brought to their attention by a member country.

Since the creation of NATO in 1949, Article 4 has been invoked six times.

Latvia, Estonia, Lithuania, Poland and other Allies agreed to hold consultations under NATO's Article 4 on Thursday morning. It comes after Russia launched its full scale invasion of Ukraine earlier this morning.

Prime Minister Arturs Krisjanis Karins of Latvia called for a consultation among NATO member nations based on Article 4, according to a Twitter post from his official account.

In conjunction, Prime Minister of Estonia Kaja Kallas said: "The most effective response to Russia's aggression is unity."

She added: "The government has prepared for this crisis, we are monitoring the situation all the time and ensuring people's safety," she said in a statement.

"Russia's widespread aggression is a threat to the entire world and to all NATO countries, and NATO consultations on strengthening the security of the Allies must be initiated to implement additional measures for ensuring the defense of NATO Allies."

https://www.express.co.uk/news/world/1571086/latvia-article-4-nato-russia-ukraine-evg