Nearly 3,000 UK troops and two Royal Navy warships are taking part in the largest Nato training exercise in more than a decade.
The major exercise is aimed at training and testing a multinational rapid response force including air, land, maritime and special forces components.
A total of 36,000 military personnel from 30 nations as well as the European Union, African Union and 12 major NGOs are involved in Exercise Trident Juncture, which is mainly taking place in Italy, Portugal and Spain with further elements happening in Belgium, Canada, Germany, the Netherlands and Norway.
The naval practice is taking place in the Atlantic and the Mediterranean.
The flagship of the Royal Navy, helicopter carrier HMS Ocean, and assault ship HMS Bulwark, both Plymouth-based ships, are taking part along with 2,800 UK troops.
During the exercise, a flotilla of warships from several nations lined up just hundreds of metres apart for a photograph to exhibit the military strength involved.
General Hans-Lothar Domrose, the commander of joint force command Brunssum, said: "The purpose of the exercise is to train and test the Nato response force, a highly ready and technologically advanced multinational force comprising air, land, maritime and special forces components.
"Enhancing our response forces is a key part of Nato's overall effort to adapt to emerging security challenges. Trident Juncture 2015 has been designed to ensure that our concepts and procedures will work in the event of a real crisis because our job is always to be prepared to defend the people, territory and values of this alliance."