WASHINGTON - The outbreak of deadly anti-China protests in Vietnam raises the stakes for the United States, which has rallied behind Beijing's neighbours but faces ugly new realities.
Demonstrations have spread to a third of Vietnam's provinces, with workers attacking Chinese workers and factories, in a wave of nationalist outrage after Beijing moved a deep-water drilling rig into waters claimed by Hanoi.
The escalation came despite months of US cajoling for an easing of tensions in the myriad disputes in the South China Sea and a separate conflict between China and US ally Japan in the East China Sea.
President Barack Obama has put a high priority on building relations with Southeast Asia, seeing the region as economically dynamic and eager for warmer US relations faced with China's rise.
Vietnam has been a case in point, readily seeking military ties with the United States in a dramatic shift for the onetime war adversaries.
"I think this poses a huge dilemma for the United States," Patrick Cronin, an Asia expert at the Center for a New American Security, said of the violence in Vietnam.
"It's not enough to have a strategic dialogue between Hanoi and Washington. We really have to reassure the Vietnamese public - not just the government - that there are going to be agreed-upon rules and that Vietnam will not be victimized."
State Department spokeswoman Marie Harf reiterated the US criticism of China's "provocative" actions but called for protests in Vietnam to be peaceful.
A setback for Vietnam
Dean Cheng, a senior research fellow at the conservative Heritage Foundation, said that the violence risked playing into the hands of Beijing, which would consider itself the aggrieved party. China has accused Hanoi of "connivance" with rioters.
"It certainly complicates American calculations because the US tends to want the people it supports to be pristinely pure and good, and attacking foreign direct investment muddies that," Cheng said.
The analyst said that the violence - also targeting businesses from Taiwan, which is autonomous from China - risked making Vietnam appear "a lot less pleasant" as an investment destination.