Last year's Exercise Joint Warrior on Salisbury Plain.
More than 10,000 military personnel from 13 countries have arrived in the UK ahead of Joint Warrior.
A total of 35 warships, five submarines and 59 aircraft, including helicopters, are also taking part in the NATO exercise.
Belgium, Canada, Denmark, Estonia, France, Germany, Italy, Latvia, Lithuania, the Netherlands, Norway, Spain, the UK, and the US are all involved.
The Ministry of Defence (MOD) say the exercise enables the UK and its allies to train together against a range of current and future threats.
Defence Secretary Gavin Williamson said: "The UK will not waver in continuing to work with its allies to protect our mutual interests.
"Led by the UK, Joint Warrior allows key NATO allies to tackle complex warfare scenarios to ensure that the international community stands fit and ready to work to face any threat together."
The exercise is being controlled from HM Naval Base Clyde.
A range of air, surface, sub-surface, sea control and maritime security roles are all being rehearsed off the west coast of Scotland.
Aircraft will operate from a number of RAF bases including RAF Lossiemouth, RAF Marham and RAF Brize Norton.
Air units will exercise Intelligence, Surveillance and Reconnaissance (ISR), air defence, air mobility and attack, according to the MOD.
WATCH: the exercise in 2018 culminated on Salisbury Plain, when the UK Joint Expeditionary Force conducted urban combat operations.
Land units are expected to focus on the integration of close air support at the Cape Wrath naval bombardment range and the RAF bombing range at Tain.
There will also be amphibious landings by Royal Marines at the MOD’s Castlemartin range in Wales.
In total, 10,300 armed forces personnel are taking part - 6,500 sea personnel, 3,000 land personnel and 800 air personnel.
The exercise runs from 30 March to 11 April.