NATO’s Allied Rapid Reaction Corps (ARRC) has been testing its ability to command tens of thousands of troops from the alliance across all domains.
Exercise Loyal Leda has been challenging the Corps' capacity to deploy around the world to tackle crises.
The ARRC Headquarters in Gloucestershire is already on 20 days notice to move anywhere in the world.
It is the first time NATO has maintained a corps-level headquarters at high readiness since the Cold War, with Exercise Loyal Leda providing the official certification for this.
"This is not a one-day exam," said Major General Lorenzo D'Addario, Deputy Commander of ARRC.
"This has actually been a journey, but very much is a test and very much is an opportunity for us to grow and also to perform to the standards that are required from the alliance."
During the training, the troops are having to show proficiency across all five military domains - air, land, sea, cyber and space.
Major Luke Reardon, from the US Air Force and Space Support Coordinator at ARRC HQ, said testing their ability to operate in space is "absolutely real life".
"As an operational domain, [space] was announced officially in NATO in just December of last year," he said.
"We’re within one year and already we’re seeing joint all-domain integration exercises proliferate down through the alliance in a coherent series of test and evaluation of which Loyal Leda and this exercise is extremely important."
Troops were spread out across the training area in Gloucestershire - a deliberate tactic used against potential hostile actors.
Nearly 25 years ago, ARRC deployed on a peacekeeping mission to Bosnia and Exercise Loyal Leda means they could again be sent on operations of a similar scale.
Lieutenant Colonel Baz Barrett, Chief Joint Air Ground Integration Centre, was deployed to Bosnia with the ARRC in 1995 said things are "massively different" now.
"You look at a corps headquarters today, [it] is all digital, it’s all software-driven, it’s all computers, as opposed to 25 years ago, [which was] very much clansman radios, big green heavy pieces of kit," he said.
The 1,200 troops involved in Exercise Loyal Leda are due to finish next week when the ARRC will be presented with its new guidon – an official symbol of its new deployability.
The soldiers are also taking part in an NHS COVID-19 test and trace pilot.
The personnel from 21 different countries are taking lateral flow antigen tests, which produce results within 20 minutes to an hour, once every three days.
Troops are required to take the test before being allowed onto the training area.