Consisting of 10 'like-minded' nations, the Joint Expeditionary Force (JEF) is a military alliance with a primary determination to maintain the security of Northern Europe.
JEF launched eight years ago and consists of mostly Nordic countries. The UK occupies a central position as the 'framework' nation of the alliance.
Prime Minister Boris Johnson hosted a summit with the leaders of JEF nations this week to discuss the war in Ukraine.
But which nations make up the alliance, and what does the Joint Expeditionary Force actually do?
The JEF is a northern European security coalition of mainly Nordic and Baltic countries, including the UK, Denmark, Finland, Estonia, Iceland, Latvia, Lithuania, the Netherlands, Sweden and Norway.
The Ministry of Defence (MOD) says the UK is "proud to be at the centre of JEF", acting as the alliance's "framework" nation.
Member nations can work collectively, in partnership with other nations, or individually across areas including security and humanitarian relief efforts.
Sean Monaghan, from the Center for Strategic and International Studies, says NATO established "framework nations" in 2014, providing the genesis of the JEF despite some members not sharing NATO membership status.
This involved "military formations within NATO, between like-minded partners that could co-operate together on military capabilities and military operations", he said.
"The Joint Expeditionary Force is the framework led by the UK."
What is the purpose of the Joint Expeditionary Force?
The MOD has said that JEF nations share the same values, act quickly and continuously, help bring stability and security in the North Atlantic, Baltic Sea Region and in the High North, and come "together to enhance each other's military skills and capabilities".
It also says that "like most military alliances, defence strengthens the bond between allies and helps build new relationships around the world. Co-operating with the international community helps secure a better, fairer and safer world."
Speaking ahead of a planned JEF summit later this week, the Prime Minister said resilience to Mr Putin's threats must "go beyond our military footing".
Boris Johnson added: "Together… we must ensure we are insulated from Russia's interference and impact on our energy supplies, economy and values."
The JEF "can act outside of the NATO framework", explained Mr Monaghan, so nations outside of NATO such as Sweden "presumably" are able to call on other JEF members to fulfill security commitments – separate to the formal obligations in other alliances.
How does the UK contribute to the Joint Expeditionary Force?
Physically, the UK contributes to JEF through personnel, equipment and capabilities drawn from the Armed Forces.
In the past, the UK's military contribution, alongside that of the other nations, has allowed JEF to conduct maritime exercises and deployments.
In 2019, JEF nations took part in BALTIC PROTECTOR, which included maritime exercises off the coast of Denmark and latterly Germany and Sweden. It culminated in the alliance working together to practice amphibious landings in Europe's Baltic states.
Nearly 2,000 military personnel from 17 vessels worked together across the deployment.
In February, the foreign ministers of JEF nations announced future military exercises in Northern Europe across land, air and sea.
Has the Joint Expeditionary Force deployed on operations?
In March last year, three members of JEF joined the UK to conduct maritime patrols in the Baltic Sea.
HMS Lancaster and HMS Westminster were joined by RFA Tiderace and vessels from Latvia, Lithuania and Estonia for a joint demonstration of the participant nations' commitment to the security and stability of the region.
It marked the first operational deployment for JEF and included intensive training with helicopters, refuelling at sea, gunnery and air defence, sailing in close formation and acclimatising to sub-zero temperatures as JEF ships operated deep in the Baltic.