Exercise Joint Warrior is a UK-led multinational war exercise that usually takes place in spring and autumn each year.
The Royal Navy, British Army and Royal Air Force are joined by forces from a number of other allied nations including the US, Denmark, Germany, France, Latvia and the Netherlands.
Joint Warrior aims to provide a complex environment in which the forces can train together, honing tactics and skills in preparation for deployment as a Combined Joint Task Force.
The latest Joint Warrior exercise began on Saturday 1 October.
Led by the Royal Navy, NATO forces are being tested from Cape Wrath and the Hebrides to the North Sea and Channel.
More than 11,000 military personnel are taking part, with over 20 ships and some submarines involved.
Joint Warrior is typically run from Clyde Naval Base, with the action mostly focused in north-west Scotland and adjacent waters.
This latest exercise, however, is being held across the UK and is being directed from RAF St Mawgan, near Newquay in Cornwall.
What happens during the exercise and what is its aim?
Over two weeks, Joint Warrior includes airborne assaults, amphibious landings, evacuations and live-fire exercises.
The exercise runs through a range of scenarios, including crisis and conflict situations, that could be realistically encountered in operations, such as territory disputes, terrorist activity and piracy.
Watch: In 2019, the then RAF's newest jets the F-35Bs joined Typhoon and Voyager aircraft in the multinational exercise for the first time.
The training involves warships, aircraft, marines and troops from the UK, NATO and allied forces.
As well as allowing participating units to sharpen their specialist roles within a larger war-style setting, it also helps nurture vital links between the UK, NATO and other allied militaries, the Royal Navy says.
A complex environment is created for the participants to train together, sharpening their tactics and skills in the process.
The scenario for each Joint Warrior is designed to reflect contemporary political tensions – such as the War on Terror and the threat posed by so-called Islamic State as well as simulate the hostilities that might result from them.
The Royal Navy says that assuring "maximum preparedness in the face of any threat" is the ultimate aim of the exercise.
Watch: In 2019, Royal Marines teamed up with the Dutch for training.
Last year, the Royal Navy refused to deny that the aircraft carrier HMS Prince of Wales encountered a Russian warship for the first time during training off the Scottish coast.
Also last year, the exercise featured the first landing of Commando Helicopter Force's Wildcat helicopters on board HMS Prince of Wales.
Joint Warrior in 2020 saw 11 nations, 6,000 personnel, 81 aircraft, 28 ships and two submarines take part.
While on the exercise in 2019, Forces News was able to witness the Royal Marines teaming up with their Dutch counterparts to train in Scotland.
Even further back, in 2015, Forces News reported from aboard the American warship USS Vicksburg during the huge multi-national exercise.