It was the kind of exercise that used to happen regularly in the deepest, darkest depths of the Cold War with the former Soviet Union.
In fact, it has been almost four decades since NATO nations practiced as a group the organizational and logistically complex task of rapidly moving troops and equipment across a potentially hostile Atlantic Ocean to reinforce Europe.
Over the last two weeks, warships, submarines and aircraft belonging to the western military alliance have war-gamed methods to keep the sea lines of communication open in the event of hostilities in Europe.
The exercise, known as Steadfast Defender 2021, has unfolded in three phases.
The newly reformed U.S. 2nd Fleet, based out of Norfolk, Virginia, was in charge of getting the troops and equipment safely between North American and European ports. Much of the activity has taken place in the waters off Portugal.
"The Atlantic, for the majority of my career, has been uncontested space," said Canadian Rear Admiral Steve Waddell, who led the maritime portion of the exercise at sea and also serves with the Americans as vice commander of the U.S. 2nd Fleet. "Since the end of the Cold War, it has been fairly benign in terms of free flow of goods, trade and information."
Because of the resurgence in great power competition in recent years, he said, NATO sees the need for its members to be prepared to work together in an Atlantic theatre that might not be so benign in the future.
Just before the coronavirus pandemic hit in February 2020, the U.S. Navy conducted its own initial training exercise which focused on organizing a convoy to aid Europe in the event of an emergency — the first time since 1986 that such an exercise had been staged.
The NATO exercise took that concept to the next level over the past two weeks, with 20 ships from 11 nations, including the Canadian frigate HMCS Halifax. Over 5,000 sailors, marines, aviators and other military personnel from across the alliance took part in the exercise, the first phase of which concluded on Sunday.
The commanders and their ships ran various real-time scenarios, including one that involved defending against submarines. That reflects NATO's focus on defending the Trans-Atlantic fibre optic cables over which much of the western world's commerce moves.
Defence analysts see those cables as a point of strategic vulnerability, especially given the dominance of the digital economy. Russia's ability to tamper with those cables on the sea bed has been a growing source of concern.
"Today's environment is multi-domain," said Waddell. "We consider not just the immediate surface of the water and the sub-surface where submarines operate. Now we consider [the domain] to be from seabed to space."
Among the other warships taking part was Britain's newest aircraft carrier, HMS Queen Elizabeth, with its complement of F-35B stealth warplanes.
The next phase of the exercise will test NATO's ability to quickly coordinate and move disembarked troops and equipment across Europe. At the same time, the alliance's very high readiness task force — made up of 4,000 troops and led by Turkey — will deploy to Romania to complete the drill.
The exercise comes ahead of a summit between U.S. President Joe Biden and Russian President Vladimir Putin. It is also taking place after Russia's recent announcement that it will permanently station a large military force on its western border and reactivate a long-dormant, Cold War-era army command to control and coordinate those forces.
Although Russia likely kept tabs on the exercise, Waddell said the NATO fleet did not have any direct communication with the Russian military during the drill.