NATO's largest military exercise in more than a decade sends "a very clear message to our nations and possible adversaries", according to alliance head Jens Stoltenberg.
The three-week exercise, which began last month, brings 36,000 troops from more than 30 NATO member-nations and partners to undergo "intensive training" across Italy, Spain and Portugal.
"NATO does not seek confrontation," Stoltenberg said. "But we stand ready to defend all allies."
On Wednesday, hundreds of troops, backed by helicopters and tanks, participated in a simulated attack against a fictitious enemy at the San Gregario training ground in northeastern Spain as part of the exercise named Trident Juncture.
NATO says forces from 12 NATO allies conducted an offensive operation involving a hostage rescue, minefield clearing and infantry attacks. It says the demonstration also included the deployment of over 500 U.S. paratroopers who flew directly from their home base at Fort Bragg, North Carolina, to parachute drop onto the training ground.
Stoltenberg said NATO has invited observers from all member nations of the Organization for Security and Co-operation in Europe (OSCE) and from other nations around the world, including Russia, to the exercise.
The exercises are also testing a new 5,000-strong "spearhead" force comprising air, maritime and special operations components as part of a 40,000-strong rapid reaction force.
The exercise began October 19 and will run through November 6. It includes more than 230 units, over 140 aircraft and more than 60 ships.