US Marines recently took on Britain's Royal Marines in a battle simulation in the Mojave Desert.
British commandos forced their US counterparts to surrender before halftime, The Telegraph said.
The Royal Marines went from controlling 20% of the battle area to more than 65%, the newspaper said.
US Marines were driven into submission by their British counterparts during a training exercise held deep in the California desert last week, according to a report in Britain's Daily Telegraph newspaper.
The paper reported the group of Royal Marines employed new tactics that helped them get the better of the American troops, who were on home soil.
According to The Telegraph, Exercise Green Dagger involved the two nations facing off against each other in a competition in which the sides were tasked with taking out one another's assets.
The Royal Marines "dominated" the US in the five-day simulation, it said, with US forces asking for a "reset" less than halfway through.
At one point, the Royal Marines had taken out or disabled almost all the US assets, The Telegraph said.
The Royal Navy told Insider that the victory was decisive.
In a statement, it said the mock battle "concluded with a last-minute 'enemy' assault which was repelled, leaving allied forces in control of over two thirds of the entire 'battlefield.'"
The navy said the British commandos "won decisive battles early on and gained ground from their enemy, but, with the US Marines pushing into allied territory, Royal Marines and their allies carried out raids behind enemy lines to stop further counterattacks."
The UK forces celebrated online in the aftermath.
A tweet on Saturday by 40 Commando, a subdivision of the Royal Marines, said UK forces triumphed in "an epic close quarters finale."
—40 Commando Royal Marines (@40commando) October 30, 2021
The US Marine Corps did not immediately respond to Insider's request for comment.
The US and UK are close allies, and their militaries train together regularly.
Exercise Green Dagger was also designed to test US Marine Corps units before any overseas deployment, according to The Telegraph.
The exercise, conducted over a 3,500-square-kilometer zone, also featured actors playing the role of civilians, The Telegraph said.
"Throughout this deployment our focus has been on integrating game-changing capabilities from across the commando force to deliver disproportionate effect in the face of a free-thinking peer adversary," Lt. Col. Andy Dow, the commander of the British force, told The Telegraph.
The Dutch marine corps was also involved in the simulation, the Royal Navy said.
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