Japan and South Korea participated as "observers" in this year's military exercise between the United States and the Philippines that started Monday for an 11-day run.
The drill was taking place with simmering tensions between the Philippines and China in the South China Sea, between the United States and China over Taiwan, and North Korea's test firings of ballistic missiles.
It marked the first time that Tokyo and Seoul have joined the annual drill between Washington and Manila.
The joint drill on Philippine islands, including those facing the South China Sea, took place almost simultaneously with another large-scale US Marines' exercise in Japan called Resolute Dragon.
On Monday, Marine officials from the participating countries kicked off the sixth iteration of "Kamandag," or venom, an annual defense drill between the United States and the Philippines.
"We certainly enjoy our partnerships and alliance with like-minded nations across the Indo-Pacific," US Rear Adm. Derek Trinque told journalists at the opening ceremony.
"And so while it's always a privilege to work bilaterally with the Philippines, with Japan, with the Republic of Korea, when we can bring our allies and partners together, when we can be part of that, then we wish to do so," Trinque said.
A total of 3,760 soldiers, the majority American and Filipino, will train on counterterrorism, amphibious operations, humanitarian assistance and disaster response, as well as chemical, biological, radiological, and nuclear defense.
Of the participants, 30 are members of the Japan Ground-Self Defense Force and 120 are of the Republic of Korea Marine Corps.
US troops and aircraft on board the Tripoli, a US Navy assault ship that docked in Manila on Sept. 27, are part of the exercise.