American forces were ‘dominated’ by the British Commandos during the drill in California.
It came after the Royal Navy’s elite strike force deployed using a new battle structure.
US forces begged for a ‘reset’ half way into the five-day exercise, after having suffered significant simulated casualties.
The Telegraph reports that at one point, the Marines’ ‘kill board’ – which showed the level of damage upon enemy equipment and units – had a tick against almost every American asset, showing it had been destroyed or rendered inoperable.
The exercise covered more than 3,500 square kilometres of mountainous and desert terrain. Come the end of the drill, supported by the Royal Artillery, Marines controlled more than 65 per cent of the area, having started with less than 20 per cent.