Nato has been central to the response to Russia's invasion of Ukraine, and its leaders have been vocal in reaffirming their commitment to Article 5 of its founding treaty - but what does that oblige member states to do and will forces from the military alliance be sent to Ukraine?
Nato general-secretary Jens Stoltenberg has been vocal in his condemnation accusing Russia of "shattering" peace in Europe.
Since the invasion began, the US and many European nations have both looked to Nato for their response and been vocal in their commitment to the alliance.
Nato (The North Atlantic Treaty Organization) is a defensive military alliance of 30 countries compromising 27 European and two North American nations, as well as Turkey.
Under Nato missions, President Joe Biden has said the US will deploy 7,000 more troops to Germany to bolster Europe's defences.
The UK and Germany have also said they will deploy more soldiers to the Baltic States and Poland via Nato.
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Nato has also activated special defence plans to give commanders in eastern Europe more freedom to react to the situation and deploy forces if needed.
An extraordinary meeting of Nato's 30 heads of state, as well as Sweden and Finland will be held on Friday to respond to the crisis.
All of this is being done with reference to Article 5, yet no troops are being sent into Ukraine, so what are Nato members obliged to do?
What is Article 5?
Nato was founded in 1949 when the US, the UK, Portugal, Norway, Netherlands, Luxembourg, Belgium, Italy, France, Iceland, Denmark and Canada signed the North Atlantic Treaty in Washington DC.
The alliance is defensive in nature, it is not supposed to orchestrate offensive operations.
ITV News International Affairs Editor Rageh Omaar breaks down what the war in Ukraine means for Nato
Central to the treaty is Article 5, which enshrined the principle of collective defence that means an attack against one ally is considered an attack against all allies.
It was invoked for the first time in the wake of 9/11, which obliged all member states to come to the US's aid.
As a consequence, a large global coalition was formed that led to the invasion of Afghanistan in 2001.
Although Ukraine has several agreements with Nato and aspires to join the alliance one day, the country is not a member and so is not protected by Article 5.
This is one of the reasons western forces have not been deployed to defend Ukraine.
However, because of Russia's unprecedented aggression, Nato nations like Romania, Poland and the Baltics (all former Soviet states) now all feel increasingly threatened by Moscow.
This is why the US has been deploying troops across Europe, in order to remind Russia it will not tolerate any invasion of a Nato nation and to reassure its allies.
President Biden is also keen to emphasise his commitment to the alliance after president Trump spent years criticising it and expressing reluctance to defend Europe.
European nations feel so threatened that Bulgaria, the Czech Republic, Estonia, Latvia, Lithuania, Poland, Romania and Slovakia all collectively triggered Article 4 of the treaty on Thursday.
Article 4 allows nations to call for urgent consultations when “the territorial integrity, political independence or security of any of the parties is threatened". An "extraordinary" meeting will be held on Friday.
It is only the sixth time it has been triggered, and it has never been triggered by so many nations at the same time.
Why are Finland and Sweden taking part in the virtual summit?
Just like Ukraine, Finland and Sweden are not members of Nato, but do have several agreements with the alliance.
They are both members of the EU, which has been actively coordinating with Nato since the invasion began and will be present at the virtual summit.
Finland and Sweden have also rejected demands made by Russia towards Nato.
One of Russia's key demands to ease tensions weeks ago was that Nato rules out expanding to any more nations.
Nato rejected this demand saying it is the right of any sovereign nation to try and join Nato, something which Finland and Sweden both agreed with.
In light of this, Mr Stoltenberg said at a press conference on Thursday he had invited Finland and Sweden to join the summit.
He said the acceptance of these invitations shows "the very strong unity with demonstration of transatlantic Nato and also our close partnership with EU, Finland and Sweden".
Despite this, both Finland and Sweden ruled out seeking to join Nato in the immediate future on Thursday.
Swedish Prime Minister Magdalena Andersson said on Thursday “Sweden has been alliance-free for an extremely long time. It has served Sweden’s interests well.” Finland's foreign minister Pekka Haavisto said they would wait until after the crisis to assess whether they needed to rethink membership but acknowledged it would need cross-party support.