Large Nato drills in the fjords of northern Norway are being carried out to hone the fighting skills of the newly expanded 32-nation military alliance.

Nato is baring its fangs in its biggest exercises since the Cold War, sending an unmistakable message to Russia that alliance members are ready to defend each other if needed.

Having watched Russia’s full-scale invasion of Ukraine, now in its third year, the Nato training aims to cover all eventualities – and that can include trying to catch troops off guard.

As part of the exercises this week, crew members aboard the French frigate Normandie, one of France’s most modern warships, were roused from sleep and scrambled to hunt down and “kill” a submarine that sneaked into cold Norwegian waters.

The submarine belongs to Germany, also a Nato member. For the purpose of the war games dubbed Nordic Response 2024, it was acting as an enemy vessel.

The Normandie crew spotted its periscope poking through the waves and sprang into action. The submarine had already “attacked” a nearby Italian ship, the aircraft carrier Giuseppe Garibaldi, scoring an imaginary torpedo hit.

The crew were determined not to let Normandie – a top-of-the-line vessel, in service only since 2020 – suffer the indignity of also being struck.

An urgent 7am call got Normandie’s commander, Capt Thomas Vuong, up from his bunk. He ordered the frigate’s submarine-hunting helicopter to be readied for flight, waking its pilot.

“We spotted its attack periscope,” Capt Vuong told The Associated Press on board Normandie in an exclusive interview.

“Then it dived again,” he said. “We were asked to hunt for it. We succeeded.”

Once airborne, the Normandie’s NH90 helicopter hovered over the waves and lowered its submarine-detecting sonar into the sea. The frigate also used its sonar, and together, they zeroed in on the sub’s position and “attacked” it in turn.

“Intelligence confirmed to us that there were no friendly submarines in the sector, so we were certain that it was an enemy submarine,” the helicopter pilot, Lt Olivier, recounted. The French navy withheld his family name for security reasons.

“So the frigate was able to fire a torpedo and destroy the submarine,” he added – but not for real, of course.

The frigate and its helicopter pinpointed the submarine with sufficient accuracy to be sure that it would not have survived had actual torpedoes been fired.

The Normandie crew of 146 mariners got no advance warning of the German sub “attack”, to test their readiness in the inhospitable environment above the Artic Circle, Capt Vuong said.

As of this week, Nato nations now also include Sweden. It formally joined on Thursday as the 32nd member, ending decades of post-Second World War neutrality.

Finland had already joined Nato in April 2023 in a historic move after decades of its military non-alignment.

In both countries, Russia’s aggression in Ukraine triggered a dramatic shift in public opinion, leading to their May 2022 applications to join Nato.

The Nordic drill in the northern regions of Finland, Norway and Sweden involves more than 20,000 soldiers from 13 nations and kicked off on Monday. It is part of wider exercises called Steadfast Defender 24. They are Nato’s biggest in decades, with up to 90,000 troops involved over several months.

They are aimed at showing the alliance can defend all of its territory up to its borders with Russia.

German submariners are more familiar than Normandie with Norway’s deep and narrow fjords and the cold Arctic waters that can complicate submarine detection, Capt Vuong said.

The drill was “extremely beneficial, because we reach a very high degree of realism and so we better prepare our teams,” he said.

“The fjords are a special environment, with a temperature profile different to what we know in the Atlantic.”

“To be able to train our teams here, against this threat, is extremely valuable and extremely stimulating,” he added.

“This is their playing field. So they know the hiding places.”