Helicopters involved in Nato's Joint Warrior exercise have landed at RAF St Athan.
Parachute regiment 2 Para Battle Group will practice assaults at training areas in Caerwent, Monmouthshire and Pembrey, Carmarthenshire.
In total, around 13,000 NATO personnel from 14 countries are taking part.

After the final with Wycombe in 2015 ended 1-1, the defender smashed home a penalty to help Southend to victory.
"It was my Stuart Pearce moment from Euro 96. He was one of my heroes growing up," said the 37-year-old.
"To come back to Southend, the club I'd supported as a boy, and finish it in that way was a dream come true."
Barrett played 354 games in two spells with the club, the last against Stevenage in the EFL Trophy in November.
The former Bristol Rovers and Gillingham defender was a member of the Shrimpers' side that won back-to-back promotions from League Two to the Championship in 2005 and 2006, as well as the promotion in 2015.
"I'd never played at Wembley, I'd played at the Millennium Stadium a couple of times while Wembley was being rebuilt and I worried I'd missed my chance," he told BBC Sport.
"But to go to Wembley with Southend (against Wycombe), and the way we won it - Joe Pigott scored with 20 seconds to go to take it to penalties.
"I took a penalty, put my foot through it and smashed it in the roof of the net. To have all my family there, it was the cherry on the cake of my career."
Barrett admitted he has been contemplating his future for the past 12 months, especially having "always been a starter" but finding himself out of the team.
"I still feel I've got more in me and could have carried on but it just felt the right time," he said. "I'm extremely privileged to play over 700 career games, in successful teams and I've got lots of great memories.
"But unfortunately it comes to an end for all of us and this is my time."
Barrett hopes to take his playing experience into coaching, adding: "That's where I see my future, maybe management one day.
"I've had a few offers, but I'm just going to weigh my options. I'm completing my Uefa A licence at the moment and hopefully a good opportunity comes up for me."
Around 1,500 troops have converged on the Vale of Glamorgan in what is believed to be one of the largest military exercises held in Wales.
[NEXT_CONCEPT]
Adam Barrett says helping Southend United win promotion via the League Two play-offs was the standout moment of his career, after retiring on Monday.