After a decade dominated by ground wars against insurgents in Iraq and Afghanistan, the drill dubbed Bold Alligator is "the largest amphibious exercise conducted by the fleet in the last 10 years," said Admiral John Harvey, head of US Fleet Forces Command.
About 20,000 US forces, plus hundreds of British, Dutch and French troops as well as liaison officers from Italy, Spain, New Zealand and Australia are taking part in the exercise along the Atlantic coast off Virginia and North Carolina.
An American aircraft carrier, amphibious assault ships including France's Mistral, Canadian mine sweepers and dozens of aircraft have been deployed for the drill, which began on January 30 and runs through mid-February.
Monday was "D-day" for Bold Alligator, with US Marines stepping on to the beach from hovercraft, near the Camp Lejeune base in North Carolina.
The American military, mindful that Marines have spent most of their time in the deserts of Iraq and the mountains of Afghanistan since 2001, said the goal was "to revitalize, refine, and strengthen fundamental amphibious capabilities and reinforce the Navy and Marine Corps role as 'fighters from the sea.'"