Around 150 British Army and Royal Air Force personnel have deployed to Finland alongside US and Finnish armed forces for Exercise Vigilant Fox.
The deployment follows the UK's joint security declaration with Finland signed in May.
More than 750 troops from the UK, USA and Finland gathered this week in Niinisalo, western Finland, to participate in the four-day high-readiness exercise.
Partner nations practised interoperability between air and land forces ahead of Finland's accession to NATO.
Minister for the Armed Forces James Heappey said: "Exercise Vigilant Fox has demonstrated the strength and interoperability of our Armed Forces with our US and Finnish allies and reaffirms our commitment to the defence and security of the Baltic Sea region."
British troops based in Estonia as part of Project Unified Stance descended into Finland in Royal Air Force Chinook helicopters.
The troops were inserted into Niinisalo to join Finland's high readiness forces. The exercise also included soldiers, armoured fighting vehicles and helicopters from the US 3rd Armoured Brigade Combat Team (The Iron Brigade).
Units from all three nations undertook a series of demanding training events, conducting offensive and defensive operations including helicopter assaults, covert landings and short-notice raids using American Black Hawks, Finnish NH-90s and UK Chinooks.
The training aims to improve integration between air and land forces, as well as interoperability between partner nations – crucial to ensuring that NATO forces can work together effectively on operations and in war.
As well as air-land integration, the exercise focused on developing command and control procedures and tactical drills using a combination of live and blank firing drills and urban operations.
Watch: Finland's military strength explained.
Colonel Jukka Nurmi, Deputy Chief of Training, Defence Command Finland, said: "Training events like Vigilant Fox are a great opportunity to learn from each other and Finland is most grateful for participation of international partners.
"For Finland, the exercise is an effective way to demonstrate the competence of our conscripts and to verify and develop the international compatibility of our own troops.
"The training and exercise activities carried out with close partners, such as the United Kingdom, are a continuation of long-term close cooperation."
This exercise comes just two weeks after RAF fighter aircraft deployed to Finland and Sweden for joint training.