The first 20 hours of a Philippine mission this month to resupply a grounded World War II-era ship exemplified everything the South China Sea could be on a good day: calm seas, warm sun and a haven for fishermen.

The following four hours, however, displayed the tensions and inherent dangers it pose, as an increasingly determined Philippines — backed by the US — resists China’s expansive claims on the vast body of water that’s teeming with marine life and natural resources.

A crucial aspect of Philippines’ pushback under President Ferdinand Marcos Jr., as tensions escalate over the fate of the grounded vessel, has been to publicize what it describes as China’s aggression and bullying. Marcos spoke to Chinese President Xi Jinping at the sidelines of the Asia-Pacific Economic Cooperation summit in San Francisco on Friday, saying he voiced his concerns on the incidents involving vessels on both sides.

Around two dozen journalists joined the Philippine military operation earlier in November to resupply the BRP Sierra Madre, a corroding vessel that Manila deliberately grounded in 1999 in the Second Thomas Shoal to serve as its lone garrison in the vast waters.

Both China and the Philippines lay claims over the shoal while at least three other neighbors also claim the larger Spratly Islands chain it nestles in.

Manila is increasingly worried that its dilapidated outpost, with gaping holes in its structure, will soon fall apart. China, over the years has been playing the long game, hoping that water and rust will take care of the issue for it, making the ship uninhabitable.

To prevent that fate, the Philippines, under an assertive Marcos has been trying to get not just food and medical supplies to the small contingent of marines stationed on the Sierra Madre, but also equipment and materials to shore up the vessel. That has raised Beijing’s ire, and it has aggressively tried to stop Manila’s resupply operations, including by using water cannon to scare off Filipino boats. Last month, vessels from the two countries collided on two separate occasions.

Last week’s mission that included Bloomberg News started in tranquil fashion. Journalists were split into groups on board three coast guard vessels tasked with escorting two smaller resupply boats. Bloomberg News joined the 40-strong crew of the 44-meter, Japanese-built BRP Sindangan.

Not much changed for a day. The waters were calm, the air humid with a whiff of the sea. Reporters and the Sindangan crew sat on plastic chairs and chatted at the ship’s back deck.

The mood changed dramatically early the following morning. Scattered lights surrounded the Sindangan in the early hours of Nov. 10. As day broke, what had appeared to be gently floating bulbs, turned out to be flotilla of Chinese boats. The Philippine convoy of five vessels was clearly outnumbered by three dozen Chinese boats.

The radar screens lit up on the Sindangan’s command room, and soon Philippine and Chinese coast guard vessels started exchanging increasingly tense radio messages.

The Philippine boats continued their journey, defying the Chinese warnings — both in Mandarin and English — of “consequences” if they didn’t stop their “illegal operation.”

Chinese vessels, that had earlier been content with shadowing the Sindangan, soon started to close in and the ship’s personnel began documenting the Chinese maneuvers using a drone.

Two China coast guard vessels began swerving and blocking in coordination with three hulking fishing boats that Manila calls “maritime militia” vessels. At one point, one of them got within 10 meters of the Sindangan.

An officer on board the Sindangan noticed black tires hanging from the hull of a Chinese coast guard ship, and thought that there might be another collision, as there had been on a previous mission on Oct. 22.

In the ship’s command center, the captain watched as Chinese fishing trawlers tried to block the Sindangan’s path. One officer took notes, other personnel were glued to their radar screens, while some scanned the waters for more menacing moves from the Chinese boats.

Two women, with “Angels of the Sea” printed on the back of their navy blue shirts, continued their pre-scripted radio responses to the Chinese: “This is Philippine Coast Guard vessel BRP Sindangan conducting a routine maritime patrol within the Philippine exclusive economic zone. We are proceeding to our planned route. Leave immediately. Stay clear from our passage.”

Amid the tensions, there were moments of levity. A radio message from the coast guard district commander, aboard another vessel, warned that another collision would give him a headache. “I’ll have to write a long report again,” echoed the radio message in the control room, eliciting chuckles among the sailors.

Meanwhile, the Philippines’ two small, military-chartered boats — ML Kalayaan and Unaizah Mae 1 — carrying the supplies for the marooned Sierra Madre troops continued to dodge the Chinese vessels, as they edged closer to the Second Thomas Shoal.

As the Kalayaan, a replacement to the boat that that had been badly damaged in the October collision with a Chinese vessel, approached the entrance of the shoal, a Chinese coast guard ship fired water canon at it, putting the nearby Sindangan crew on edge.

While the Kalayaan, which translates to freedom in the local language, managed to pull through after the Chinese water cannon and headed towards the Sierra Madre, a US Navy patrol aircraft appeared over the disputed shoal.

Philippine military spokesperson Colonel Medel Aguilar said Washington, Manila’s defense ally, has been providing technical assistance. “In this particular operation, the US aircraft allowed us to enhance our maritime domain awareness,” he said.

The supply boats reached the entrance of Second Thomas Shoal just after 8:15 a.m. that Friday. Using rubber boats, provisions were delivered to the Sierra Madre, as bigger Philippine and Chinese ships that couldn’t enter the shoal’s shallow waters, watched from afar.

Within hours, recriminations were flying, as both Manila and Beijing issued statements accusing each other of aggression and Manila said its embassy in Beijing formally protested the water cannon incident.

The Philippines declared the resupply mission a success despite over three hours of confrontation with Chinese ships, which revealed an obvious challenge for Manila as it pushes back against a much powerful rival: China’s maritime presence easily dwarfed the Philippine contingent.

China had a fleet of 38 — 28 fishing vessels, five coast guard boats, five military vessels including a fast attack craft and even a hospital ship. The Philippines deployed three coast guard vessels and two resupply ships.

President Marcos told reporters after meeting Xi that communication between the Philippines and China is key, and the two sides will try to come up with mechanisms to ease tensions.

“We have to continue to be candid with one another and to be sincere in our desire to keep the peace,” Marcos said. “That sincerity exists for all parties involved. I do not think anybody wants to go to war.”

--With assistance from Yuki Tanaka, Elaine To and Manolo Serapio Jr.