Spurred by Russia's aggression on Ukraine, Sweden and Finland have both formally requested to join NATO and if accepted would leave to just four the number of European Union member states not in the trans-Atlantic military alliance.
These are Austria, Cyprus, Ireland, and Malta.
All four are either militarily neutral, which means they cannot join a military alliance or take sides in military conflicts, or non-aligned, meaning they do not officially favour one major power bloc.
In the case of Cyprus and Ireland, neutrality was historically based on the fact that the two islands are split into two separate territories.
In the case of the island of Ireland, it is divided between the Republic of Ireland and the British province of Northern Ireland with an open border between the two polities that resulted from the 1998 Good Friday peace agreement that ended decades of sectarian violence.
Cyprus, however, is divided between the Republic of Cyprus in the south and the self-declared Turkish Republic of Northern Cyprus which is not recognised by the international community.
Yet all have ties to NATO. Austria, Ireland and Malta take part in the alliance's Partnership for Peace Programme which provides a framework for enhanced political and military cooperation for joint multilateral activities, such as humanitarian assistance, peacekeeping, and crisis management.
Cyprus, meanwhile, has two British military bases on its soil with the UK being a NATO member.
Russia's invasion of Ukraine on 24 February has ignited debates in most of Europe's neutral countries over whether such a status is still warranted.
Sweden and Finland, for instance, were both traditionally neutral, but public opinion shifted swiftly in favour of NATO membership when Moscow sent its tanks into Ukraine.
Several other European countries have also not joined the alliance. These include Armenia, Azerbaijan, Bosnia and Herzegovina, Kosovo, Moldova and Serbia as well as Switzerland.
The latter is perhaps the world's most famous neutral country with the first mention of the status dating back to 1515 although it was formally established in 1815 following the defeat of Napoléon Bonaparte in Waterloo.
It then signed The Hague Conventions of 1907 which codified neutrality as part of international law, stipulating that neutral countries must refrain from engaging in war, ensure equal treatment for belligerent states in respect of the exportation of war material, not supply mercenary troops to belligerent states, and not allow belligerent states to use its territory.