Russia is threatening to respond to the NATO exercise (Picture: NATO).
NATO has launched its biggest war games since the end of the Cold War.
Trident Juncture involves around 50,000 soldiers from 31 countries, with the aim to send a clear message to any potential adversary.
It also includes 65 ships, 150 aircraft and 10,000 vehicles.
However, Russia has been angered by how close the show of strength is to its border.
NATO troops will be working together to repel a mock invasion of Norway.
Russia has been invited to monitor the drill - but has been issuing threats instead.
Moscow says it could be forced to respond to increased NATO activity near its border with Norway.
The UK's contingent will be 2,700-strong and mainly made up of Army personnel. The UK is also contributing 480 vehicles and six ships.
NATO Secretary-General Jens Stoltenberg said it is a chance to practise responding to threats together:
"This is a strong display of our capabilities and of our resolve to work together.
"The scenario is fictitious, but the lessons we learn will be real."
Russian defence minister warned that Kremlin could be forced to respond to increased NATO activity close to its western border.
"Nato's military activities near our borders have reached the highest level since the Cold War times," Sergei Shoigu said, noting that the war games will be "simulating offensive military action".
During Exercise Trident Juncture, participants will be split into 'north' and 'south forces, taking turns playing the role of the fictitious aggressor and the NATO defending forces.
The exercise will test the alliance's readiness to restore the sovereignty of the ally, in this case, Norway, after an act of armed aggression.
Its aim is to ensure that NATO forces are trained, able to operate together, and ready to respond to any threat from any direction, according to the 29-member alliance.
Mr Stoltenberg added: "Trident Juncture sends a clear message, to our nations, and to any potential adversary.
"NATO does not seek confrontation, but we stand ready to defend all Allies, against any threat."
Exercise Trident Juncture 18 live field exercise runs from 25 October to 7 November and a command post-exercise runs from 13 to 24 November.