U.S. efforts to promote peace and stability in the South China Sea are facing a new challenge. This time, the difficulty comes not from China but from the leader of a U.S. treaty ally – President Rodrigo Duterte of the Philippines.

In recent weeks, the U.S.-Philippine alliance has come under strain as Duterte has rebuked the United States and threatened drastic changes in Philippine foreign policy. His volatile behavior threatens the alliance, President Obama’s strategy for “rebalancing” to Asia and the stability of the Southeast Asian strategic landscape.

How is incendiary rhetoric like Duterte’s likely to affect a strong defense partnership and regional security more broadly? This is the kind of question my research on the international relations of the Asia-Pacific addresses.

Duterte’s outbursts

Since taking office in late June, Duterte has launched a ruthless domestic war on drugs and declared that he doesn’t “care about human rights.” Those critical of his policies have met with his sharp, uninhibited tongue. “F-ck you,” he most recently told his critics in the European Union.

Senior officials from the United States, a treaty ally since 1951, have not been spared. Even mild U.S. criticism has irritated Duterte’s thin skin, prompting him to describe U.S. Secretary of State John Kerry as “crazy” and call President Barack Obama a “son of a whore.” He has chided the United States as a former colonial power, announced plans to expel U.S. special forces engaged in counter terrorism training, halted joint patrols in the South China Sea and said he would consider buying arms from China and Russia.

The line between Duterte’s bombast and real policy views is unclear. He has already backtracked from his pledge to expel U.S. special forces and said the Philippines needs the United States in the South China Sea. Still, his volatility threatens the U.S.-Philippine alliance, the strongest check against unilateral Chinese expansion in the South China Sea.

Reorienting the Philippines?

Duterte’s pledge to recalibrate Philippine foreign policy is not surprising.

Under his predecessor Benigno Aquino, the feud between China and the Philippines in the South China Sea intensified. Both states prize the sea’s hydrocarbon deposits, large fisheries and vital shipping lanes. The dispute over sovereignty also activates nationalist sentiment, as Cornell professor Jessica Chen Weiss and I stress in recent research. China’s launch of an island-building campaign in the South China Sea added fuel to the fire.

As tension in the South China Sea rose, the U.S.-Philippine alliance strengthened to a level not seen since the Cold War. Arms sales, joint exercises and training increased, and a 2014 agreement gave U.S. troops extended access to Philippine military facilities.

Not all Filipinos approved. Some derided the added U.S. military presence as an affront to sovereignty that would alienate Beijing without providing effective protection. As I argued in my book “The Limits of Alignment,” fears of losing autonomy and antagonizing foreign rivals are the most common reasons why Southeast Asian governments seek to limit their strategic relationships with great powers like China and the United States. Some analysts thus welcomed early indications that Duterte would pursue more “balanced relations” with China and the United States.

What has alarmed diplomats and strategists is the bold, inconsistent and temperamental nature of Duterte’s foreign policy pronouncements. At times his reputation for impulsive speech is helpful diplomatically. It enables embarrassed Philippine officials and nervous international partners to downplay his remarks as hyperbole, as his aides did after he threatened to withdraw from the United Nations in August. Still, his comments have to be taken seriously. Many observers discounted Duterte’s campaign promises to employ brutal anti-drug tactics. With more than 3,500 suspected drug dealers killed since the start of July, his remarks were clearly more than electoral bluster.

Discord between Duterte’s rhetoric and the views of the security establishment add further uncertainty. In mid-September, as Duterte railed against American colonial abuses, the head of the Philippine armed services insisted that the alliance is “rock solid.” The possibility of abrupt moves or domestic fracture in the Philippines raise real dangers to regional stability.

As Defense Secretary Ash Carter has stressed, a strengthened U.S.-Philippine alliance is a key to U.S. engagement in Southeast Asia. It represents a means for projecting American force and maintaining a larger regional presence over time. Just as importantly, it is a vehicle for demonstrating U.S. credibility and commitment to the region as China’s waxing power and assertiveness test U.S. resolve.

Duterte’s threats to expel U.S. special forces, cease joint patrols, and buy arms from China greatly complicate the implementation of the 2014 U.S.-Philippine defense agreement.

The United States does have other means to maintain a strong naval presence in Southeast Asia if cooperation stalls. For example, the U.S. regularly rotates naval warships through Singapore. But the United States has a clear disinterest in seeing one of its oldest Asian allies turn down American military protection in favor of direct talks with Beijing over the South China Sea. As a result, U.S. officials have been loath to criticize Duterte’s domestic abuses or foreign policy plans too sharply.

Chinese responses

For its part, the Chinese government has welcomed Duterte’s pronouncements cautiously.

In state-owned media, Chinese analysts initially lauded Duterte’s plans to abandon Aquino’s “lopsided” and “unscrupulous” China policy but later remarked on his “reckless comments” and the Philippines’ “uncertain future” under his leadership. A September meeting between Duterte and Chinese premier Li Keqiang yielded cordial pledges to pursue better ties, but a breakthrough was not clearly in sight.

Chinese officials seem to recognize that volatile leaders can swing in more than one direction. Moreover, anti-China nationalist sentiment is widespread in the Philippines and among the military leadership, constraining Duterte’s ability to compromise. His high approval ratings – 91 percent at the end of July – do not ensure that his harsh domestic policies or swings in foreign policy will not come back to haunt him.

Regional repercussions

Despite Duterte’s domestic constraints, his embrace of bilateral talks is a boon for Beijing. China prefers to deal with smaller rival claimants over the South China Sea one by one. By contrast, the Philippines, Vietnam and several other Southeast Asian states have tried to “internationalize” the South China Sea dispute by involving the United States and “multilateralize” it by discussing it in regional forums where weaker states can pool their weight.

Vietnam will suffer if Duterte spurns the United States and bypasses multilateral talks. Vietnam needs a strong U.S. security presence as a counterweight to China. However, the ideological gap and legacy of mistrust between Hanoi and Washington prevent Vietnam from hosting robust U.S. forces itself. If the Philippine-U.S. alliance weakens, Vietnam will have little choice but to adopt a more accommodating posture toward Beijing.

The Association of Southeast Asian Nations and its members will also suffer indirectly as Duterte’s shift undermines efforts to manage the South China Sea issue multilaterally. These currents will be difficult to reverse.

The adverse consequences of Duterte’s approach may be greatest in the Philippines. With weak independent naval capacity and growing economic dependency on China, Philippine leverage in the South China Sea talks is limited. Concessions could prove explosive domestically. Should Duterte alienate the United States and embrace China, he may well arouse domestic pressure to take a hard line toward Beijing without the means to enforce one – a dangerous position at home and abroad.

While Duterte has planted the seeds of instability, he need not let them grow. The U.S. government and other Philippine partners will likely be willing to treat his recent remarks as products of a bad temper and a populist political campaign. That gives Duterte a second chance to expound a more calculated, pragmatic and well-scripted foreign policy.