The U.S. Navy’s Pacific Fleet is gearing up for the biennial Rim of the Pacific exercise — better known as RIMPAC — set to begin next month.

RIMPAC is the world’s largest recurring naval warfare exercise. On Tuesday the U.S. Navy announced that approximately 40 warships, three submarines, over 150 aircraft and more than 25,000 personnel will participate in RIMPAC 2024, which is scheduled to take place from June 26 to Aug. 2 in and around the Hawaiian Islands.

The exercise takes place as international tensions in the Pacific have led to increasing confrontations at sea, particularly in the South China Sea, a critical waterway that more than a third of all international trade travels through. Beijing has claimed the entire sea as its exclusive maritime territory over the objections of neighboring countries, and used increasingly aggressive tactics to assert its claims.

Amid these tensions, the United States has sought to bolster its Pacific alliances as it competes with China for influence.

“To promote a free and open Indo-Pacific, Exercise RIMPAC is the premier joint and combined maritime exercise, utilizing and preserving a world class maritime training environment,” the Navy said in a media release. “With inclusivity at its core, RIMPAC fosters multi- national cooperation and trust, leverages interoperability, and achieves respective national objectives to strengthen integrated, prepared, coalition partners.”

RIMPAC 2024 is the 29th iteration of the exercise since it began in 1971. According to a Navy news release, this year’s exercise is expected to include military personnel from Australia, Belgium, Brazil, Brunei, Canada, Chile, Colombia, Denmark, Ecuador, France, Germany, India, Indonesia, Israel, Italy, Japan, Malaysia, Mexico, the Netherlands, New Zealand, Peru, South Korea, the Philippines, Singapore, Sri Lanka, Thailand, Tonga and the United Kingdom.

Local activists in Hawaii have objected to the past participation by Israeli military personnel in RIMPAC and criticism has intensified amid the ongoing conflict in Gaza. The participation of Indonesian forces at RIMPAC has also faced local scrutiny owing in part to human rights concerns in the ongoing conflict in West Papua.

In the Pacific, clashes have increasingly broken out in disputed waters claimed by China and the Philippines. A 2016 international court ruling in favor of the Philippines found that China’s claims had “no legal basis,” but the Chinese military has doubled down and built bases on disputed islands and reefs, and has frequently harassed and attacked Philippine fishermen and maritime workers.

In recent months Chinese ships have escalated those efforts, attacking with water cannons and pushing farther into waters claimed by the Philippines. Recently Philippine fishermen and activists organized a flotilla of about a 100 small outrigger fishing boats to deliver supplies for fellow fishermen near disputed Scarborough Shoal, but were turned back by Chinese ships.

On Monday Philippine officials accused China of causing severe environmental damage around the Scar­borough Shoal. Philippine National Security Council Assistant Director General Jonathan Malaya announced that his government is collecting evidence to file a case against China for destroying coral reefs and engaging in other alleged illegal activities, such as harvesting endangered giant clams.

The Philippines has sought to tighten ties with the United States, Japan, Australia and others as it seeks to hold the line against what it considers Chinese encroachment. The Philippines is one of several RIMPAC participants with ongoing territorial disputes with China, many of which have also sought closer military ties with the U.S. in recent years.

But though tensions with neighboring countries are increasingly strained, China has also sought to bolster alliances with investments across the Pacific.

For the past five months two Chinese warships have been docked at Cambodia’s Ream Naval Base, which was recently expanded with Chinese funding. China and Cambodia are currently holding a joint military exercise. Beijing has also been investing heavily in Pacific Island nations, including signing a controversial secret security agreement with the Solomon Islands in 2022, and expanding deployments of police advisers across Oceania.

In Hawaii, RIMPAC planners have increasingly sought to bring participants from other countries into leadership roles in the exercise. Chilean Navy Commodore Alberto Guerrero will serve as deputy commander of the exercise combined task force, Rear Adm. Kazushi Yokota of the Japan Maritime Self-Defense Force will serve as vice commander, Canadian Royal Navy Commodore Kristjan Monaghan will command maritime forces and Australian Air Commodore Louise Desjardins will command aircraft.

The overall commander of the exercise is traditionally the commander of the U.S. Navy’s San Diego-based 3rd Fleet. Currently that’s Vice Adm. Michael Boyle, who commanded the 2022 iteration of the exercise. But earlier this month the Pentagon announced that President Joe Biden has nominated Vice Adm. John Wade, former commander of the joint task force that oversaw removal of most of the fuel in the Navy’s Red Hill facility, to take over 3rd Fleet.

Depending on when and if Congress approves Wade’s nomination, it’s possible that he will oversee the exercise in Hawaii as the military continues to try to regain public trust after fuel from the Red Hill facility tainted the Navy’s Oahu waterline, which serves 93,000 people, in November 2021, causing thousands to report symptoms ranging from extreme rashes to vomiting and neurological problems. The Navy is now working to permanently shut down and remediate the Red Hill facility, which sits just 100 feet above a critical aquifer that serves most of Honolulu.

Earlier this month a trial in a federal lawsuit brought by families affected by the water crisis who report long-term symptoms from fuel exposure concluded in Honolulu’s district court, with 17 plaintiffs asking for between $225,000 and $1.25 million to each person depending on the degree of suffering and injuries each endured. Presiding Judge Leslie Kobayashi is expected to issue a ruling on damages sometime this summer.

The fuel reserve in Red Hill previously played a role during RIMPAC as the world’s navies congregated in Hawaii. During RIMPAC 2018, the Red Hill facility provided over 19 million gallons of fuel to participating U.S. and foreign ships and aircraft, according to a Navy news release at the time. The last time it was used for the exercise was in 2020.

The military brought in chartered commercial tankers to ferry the Red Hill fuel to West Oahu facilities run by Island Energy Services at Campbell Industrial Park, to a fuel storage point in San Diego, a fuel storage point in the Philippines at Subic Bay and another fuel storage point in Singapore as part of a new “distributed” fuel storage model. After decades of insisting Red Hill was critical, the Pentagon now says this new strategy provides more flexibility and more “resilient” supply lines in the event of a conflict or crisis in the Pacific.