Exercise Defender Europe 21 includes more than 30,000 personnel from 27 nations who will conduct co-ordinated operations across more than 30 training areas in a dozen countries.
The British Army will be playing a leading role in the exercise which will act as a validation of NATO's ability to deploy large-scale troops and equipment in defence of the Euro-Atlantic area, the Army said.
The UK Armed Forces' ability to integrate with allies as part of a wider force will also be tested.
Brigadier Jo Chestnutt, Commander 104th Logistic Support Brigade, the British Army's most senior officer involved, detailed a number of key exercises underpinning Defender 21.
"We've got infantry from 3rd Battalion The Rifles involved in Exercise Noble Jump, in Romania, a company group from 2nd Battalion The Parachute Regiment taking part in Exercise Swift Response, as well as soldiers from the Allied Rapid Reaction Corps involved in the Defender Command Post Exercise, a planning exercise spread between Germany and Romania," he said.
"Specifically for 104 Brigade, we're the theatre sustainment brigade, able to position ourselves to be able to deliver material through, with and in support of other partner nations."
He also explained why taking part in the training is "so vitally important".
"Firstly, it reinforces our unity and our bond," Brig Chestnutt said.
"Secondly, it proves we're combat-ready and able to test those circuits and understand exactly what it means to be positioned forward to get access to the southern European theatre whenever that time of need comes.
"Lastly, it's about showing a willingness to our partners and less about systems and processes, but more towards friendships and bonds which are forged now, but matter when the time comes."
Training exercises linked to last year's exercise, Defender Europe 20, were cancelled due to the coronavirus pandemic, though the main exercise itself did continue.
It was said to be the biggest deployment of US troops to Europe for an exercise in 25 years.
Cover Image: British Army soldiers take part in Exercise Defender Europe 21, the biggest deployment of multinational troops in Europe for decades (Picture: MOD).