Nato has commenced its largest exercise since 2002, Trident Juncture, bringing armed forces from more than 30 allied nations together.
Being conducted in Italy, Portugal and Spain, the five-week exercise will see the participation of around 36,000 international troops, including 230 military units, 140 aircraft, and more than 60 ships.
UK Military representative to Nato and EU vice-admiral Ian Corder said: "Trident Juncture is the biggest and most ambitious Nato exercise in more than a decade, focussed on exercising crisis response, collective defence, and helping to shape the future adaptation and posture of the Alliance.
"It will demonstrate the Alliance's capability, readiness, and ability to address whatever threats it may face, now and in the future."
Trident Juncture will test the Nato Response Force (NRF), demonstrating Nato's readiness, flexibility and capability of responding to threats from any direction.
The two-part exercise is also aimed at testing the functions of the high-readiness joint task-force (Spearhead Force) before it becomes fully operational next year.
According to the UK Ministry of Defence, all three military services of the country, namely the Royal Navy, Army and Royal Air Force, are participating in the exercise.
The UK sent around 2,800 personnel to the drill, including an army brigade headquarters and battlegroup, three royal navy warships and aircraft, including Typhoon fighter jets and helicopters.
Defence Secretary Michael Fallon said: "The exercise will help preparations for Nato's new rapid reaction taskforce, which the UK will provide troops for every year into the next decade and which we will lead in 2017."
Exercise Trident Juncture will also enable the Royal Navy's Maritime Battle Staff to command the naval element of the Nato Response Force in 2016.
Image: Exercise Trident Juncture is the largest Nato military exercise since 2002. Photo: courtesy of Crown Copyright.