With beach landings, 25 naval ships and an air assault, the United States and eight other countries are staging a major amphibious exercise on the US East Coast this week, fighting a fictional enemy that bears more than a passing resemblance to Iran.

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.'"

With defense spending coming under pressure after years of unlimited growth, the Marines - which devoted a brigade to the exercise - also are anxious to protect funding for their traditional role as an amphibious force.

The exercise scenario takes place in a mythical region known as ‘Treasure Coast,’ with a country called Garnet, a theocracy, invading its neighbor to the north, Amberland, which calls for international help to repel the attack.

Garnet has mined several harbors and deployed anti-ship missiles along the coast.

The threat of mines, anti-ship missiles and small boats in coastal waters conjure up Iran's naval forces, but the commanders overseeing the drill, Admiral Harvey and Marine Lieutenant General Dennis Hejlik, say the scenario is not based on any particular country.

Amid rising tensions with Iran and threats from Tehran to close the strategic Strait of Hormuz, naval officers and military planners are keenly aware of the Islamic Republic's arsenal of mines and anti-ship missiles.

When asked by reporters last week, Harvey acknowledged that the exercise scenario was "certainly informed by recent history" and that it was "applicable" to the Strait of Hormuz, as well as other areas.

Harvey also said the exercise incorporated lessons from the 2006 Lebanon conflict, when Iran-backed Hezbollah forces hit an Israeli navy corvette with an anti-ship missile.

The Pentagon opened the drill to allied forces for the first time this year, with 650 French troops among those participating.

In their AMX-10 wheeled reconnaissance vehicles and VAB armored personnel carriers, the mission of the French forces was "to land first to secure a path for the Americans," said Second Lieutenant Chens Bouriche, a French military.