Just 4 EU states could be left out of the alliance

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 favor 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.

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 favor of NATO membership when Moscow sent its tanks into Ukraine.