Sweden and Finland are on the cusp of joining Nato after the alliance formally invited them to join.
Further steps such as domestic ratification will now be taken to make the two nations' position official.
It comes after Turkey's president Erdogan confirmed his country was officially lifting its objections against the countries' membership ahead of a gathering of Nato members in the Spanish capital Madrid.
At the summit, the alliance agreed a “fundamental shift” which will see it return to Cold War-style readiness to respond to the increased threat posed by Russia.
The move to join Nato marks a major change in foreign policy from Finland and Sweden following Russia's invasion of Ukraine.
What is Nato?
Nato, or the North Atlantic Treaty Organization to give it its full title, is a multi-nation military alliance set up in 1949 in the aftermath of the Second World War.
Currently made up of 30 countries, it was designed to bring peace by guaranteeing the security of its members through military and political means.
The alliance between countries from Europe and North America gives the two continents a link which enables them to consult and cooperate in the field of defence and security.
One of the key principles and cornerstones of the alliance is Article 5, which states an attack on one member state constitutes an attack on all member states.
Article 5 has only been invoked once — in 2001 following the Twin Towers attacks in New York on 11 September.
Why are Sweden and Finland joining Nato?
Finland had been officially neutral since signing a pact with Russia in 1948, agreeing to never join a military alliance hostile to Russia, or allowing its territory to be in an attack against Russia.
Sweden has not been at war since the Napoleonic era and has built its security policy on "non-participation in military alliances".
For both countries, the invasion of Ukraine, which Moscow calls a "special military operation", has forced a radical rethink.
The two nations are vulnerable due to being geographically close to Russia, with Finland sharing an 830-mile land border.
Their acceptance into Nato is likely to infuriate Vladimir Putin, who has spent decades viewing Nato as a threat.
Moscow has been clear that it opposes any chance for the alliance to get larger.
Last month, Putin said he would have "no problem" with the two nations joining — in a break away from previous rhetoric surrounding the topic — but warned an "expansion of military infrastructure into this territory would certainly provoke our response."
Watch: Nato Secretary-General announced 'new strategic concept' at Madrid summit
Since the invasion of Ukraine, Nato countries have been supplying weapons to Ukraine to defend itself, including next generation light anti-tank weapons (NLAWs) — shoulder-mounted guided missiles which have proven to be useful against tanks.
More than 700 Switchblade drones — small enough to be carried in a backpack — and 121 Ghost drones have also been provided.
Which countries are in Nato?
At the time of its creation, there were 12 founding members:
Since then a further 18 have joined, with Sweden and Finland soon to be added to the list:
the Czech Republic