South Korea's air force says the drill involved a B-52 bomber from the United States and fighter jets from South Korea and Japan.
Despite the name, the 12th Marine Littoral Regiment will be only the second regiment of its kind.
A birdwatcher in Japan reportedly found an American artillery shell, then waited a week to report it.
Tokyo and Washington say the relocation within Okinawa, Japan, instead of moving it elsewhere as demanded by many Okinawans, is the only solution.
:quality(70)/cloudfront-us-east-1.images.arcpublishing.com/archetype/DXXPMTADBFAZZMSXUA2PFLHIR4.jpg)
:quality(70)/cloudfront-us-east-1.images.arcpublishing.com/archetype/SP2MZI6KAVAFZIUG4R732VU7L4.jpg)