Amid a simmering trade war, the US and Chinese militaries have exchanged tit-for-tat measures with each other in and above the South China Sea.

In early October 2018, a US Navy destroyer sailed close to Chinese-occupied territory in the area, a freedom-of-navigation exercise meant in part to contest Beijing's expansive claims.

During that exercise, a Chinese destroyer approached the US ship — reportedly as close as 45 feet — in what Navy officials called an "unsafe and unprofessional maneuver."


"The tension is escalating, and that could prove to be dangerous to both sides," a senior US official told Reuters on Sept. 30, 2018, after China canceled a meeting between its officials and Defense Secretary Jim Mattis — the second senior-level meeting called off in a week.

The encounter between the US and Chinese ships took place near the Spratly Islands, at the southern end of the South China Sea. Farther north, at Scarborough Shoal, the US, the Philippines, and China have already butted heads, and their long-standing dispute there could quickly escalate.

The Philippines took over Scarborough after its independence in 1946. But in 2012, after a stand-off with the Philippines, China took de facto control of the shoal, blocking Filipino fishermen from entering.

Map showing territory claimed by the Philippines, including internal waters, territorial sea, international treaty limits, and exclusive economic zone.

Chinese control of Scarborough — about 130 miles west of the Philippine island of Luzon and about 400 miles from China's Hainan Island — is an ongoing concern for the Philippines and the US.

Given the shoal's proximity to the Luzon, if "China puts air-defense missiles and surface-to-surface missiles there, like they have at other South China Sea islands, they could reach the Philippines," Bryan Clark, a senior fellow at the Center for Strategic and Budgetary Assessments, said in late August 2018.

That would be "the most direct sort of pushback on the Philippines' attempt to assert control over Scarborough Shoal," said Clark, a former US Navy officer.

Beyond a challenge to Manila, a military presence on Scarborough could give China more leverage throughout the South China Sea.

Scarborough would be one point in a triangle edged by the Spratlys and the Paracel Islands, both of which already house Chinese military outposts.

While China can use shore-based assets in the air-defense identification zone it declared over the East China Sea in 2013, the eastern fringe of the South China Sea is out of range for that, Clark said.

"So their thought is, the Chinese would really like to develop Scarborough Shoal and put a radar on it so they can start enforcing an ADIZ, and that would allow them to kind of complete their argument that they have control and oversight over the South China Sea," Clark said.

Given Scarborough's proximity to bases in the Philippines and the country's capital, Manila, as well as to Taiwan, a presence there would extend China's intelligence-gathering ability and maritime-domain awareness, said Greg Poling, director of the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies.

"But above and beyond the military implications ... China has a political interest in establishing control over all the waters and airspace within the nine-dash line, in both peace and war," Poling said in an email, referring to the boundary of China's expansive claim in the South China Sea.

'What is our red line?'

After 2012, Manila took its case to the Permanent Court for Arbitration at The Hague, which ruled in favor of the Philippines in July 2016, rejecting China's claims and finding that Beijing had interfered with Philippine rights in its exclusive economic zone, including at Scarborough. (EEZs can extend 230 miles from a country's coast.)

Ahead of that ruling, the US detected signs China was getting ready to reclaim land at the shoal, and then-President Barack Obama reportedly warned Chinese President Xi Jinping of serious consequences for doing so, which was followed by China withdrawing its ships from the area.

President Barack Obama and Vice President Joe Biden talk with Vice President Xi Jinping of the People's Republic of China and members of the Chinese delegation following their bilateral meeting in the Oval Office, Feb. 14, 2012.

(Official White House Photo by Pete Souza)

That warning was followed by increased Pentagon activity in the region, including flying A-10 Thunderbolts, which are ground-attack aircraft, near Scarborough a month later.

Tensions between China and Philippines eased after the ruling was issued, however, as Philippine President Rodrigo Duterte, who took office in July 2016, pursued rapprochement.

The Philippines said in February 2017 that it expected China to try to build on the reef, which Manila called "unacceptable." The following month, Chinese authorities removed comments by an official about building on Scarborough from state-backed media, raising questions about Beijing's plans.

More recently, the Philippines warned China of its limits at Scarborough.

"What is our red line? Our red line is that they cannot build on Scarborough [Shoal]," Philippine Foreign Secretary Alan Peter Cayetano said in May 2018.

Cayetano said the other two red lines were Chinese action against Philippine troops stationed at Second Thomas Shoal in the Spratlys and the unilateral exploration of natural resources in the area. He said China had been made aware of the Philippine position and that Beijing had its own "red line" for the area.

In July 2018, the acting chief justice of the Philippine supreme court, Antonio Carpio, said Manila should ask the US make Scarborough an "official red line," requesting its recognition as Philippine territory under the US-Philippine Mutual Defense Treaty, which obligates each to come to the aid of the other in case of attack.

"Duterte himself has reportedly said that Chinese construction of a permanent facility at Scarborough would be a red line for the Philippines," Poling said.

The Philippines' "one real option" to try to prevent Chinese construction on Scarborough would be to invoke that defense treaty, Poling said.

President Rodrigo Duterte and President Xi Jinping shake hands prior to their bilateral meetings at the Great Hall of the People in Beijing, October 2016.

It's not clear if the treaty applies to the shoal, Poling added, "but the treaty definitely does apply to an attack on Filipino armed forces or ships anywhere in the Pacific."

"So Manila would probably need to send Navy or Coast Guard ships to interfere with any work China attempted at Scarborough ... and then call for US intervention should China use force."

That could cause China to back off, as Obama's warning in 2016 did, Poling said.

While China has pulled back from previous attempts to build on the shoal, "they've got ships floating around the area just waiting for the chance," Clark said in late August 2018. "So I wouldn't be surprised if China tries to restart that project in the next year to ... gauge what the US reaction is and see if they can get away with it."

That would almost certainly force the hand of the US and the Philippines.

"If China's able to start building an island there and put systems on it, and the Philippines doesn't resist ... all bets are off," Clark said. "China feels emboldened to say the South China Sea is essentially a Chinese area."

This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.