Neither the U.S. nor China is a member of the Association of Southeast Asian Nations, but each has strong supporters in the 10-member group. ASEAN summit host Cambodia, an ally of China, has tried to shift the focus to economic concerns, but Beijing's territorial disputes with countries including U.S. ally the Philippines have overshadowed discussions.
The disagreement sparked a tense moment Monday at the summit when Philippine President Benigno Aquino III challenged Cambodian Prime Minister Hun Sen, who had tried to cut of discussion of territorial disputes.
An expanded meeting called the East Asia Summit involving all ASEAN countries and eight other nations, including China and the United States, will be held Tuesday in Phnom Penh.
Obama was expected to reiterate during the summit that Washington takes no sides in the territorial disputes but will not allow any country to resort to force and block access to the South China Sea, a vital commercial and military gateway to Asia's heartland. Washington has also called for the early crafting of a "code of conduct" to prevent clashes in the disputed territories but it remains unclear if and when China would sit down with rival claimants to draft such a legally binding nonaggression pact.
The potentially oil- and gas-rich South China Sea islands and waters are contested by China, Taiwan and four ASEAN members—Brunei, Malaysia, the Philippines and Vietnam.
Vietnam and the Philippines, backed by Washington, have been raising the issue before major international forums, and want China to negotiate with the other claimants as a group. China wants one-on-one negotiations—which would give it advantage because of its sheer size and economic clout—and has warned Washington to stay away from an issue it says should not be "internationalized."
There have recently been several standoffs involving boats and other shows of force, particularly between China and the Philippines. The battle for ownership of the Spratly Islands in one section of the South China Sea has settled into an uneasy stand-off since the last fighting, involving China and Vietnam, that killed more than 70 Vietnamese sailors in 1988.
But fears that the conflicts could spark Asia's next war have kept governments on edge.
The latest diplomatic confrontation occurred a few hours before Obama touched down on Monday in the Cambodian capital, when Hun Sen announced as he was closing a meeting that all ASEAN leaders have struck an agreement to limit discussions of the divisive issue within the 10-nation bloc's talks with China.
Alarmed, Aquino raised his hand, stood up and objected to Hun Sen's statement, saying his country, which plans to bring the disputes before a U.N. tribunal, was not a party to any such agreement. It was a blunt gesture in the usually servile ambiance of the conservative bloc, an unwieldy collective of rigid, authoritarian regimes and nascent democracies.
After a brief lull, Hun Sen recovered and said Aquino's remarks would be reflected in the record of the meeting. Still, Cambodian and Chinese officials insisted that the agreement stood.
An objection from the Philippines, or any ASEAN nation, ought to be enough to thrash any agreement because the bloc decides by consensus, meaning just one veto from any member kills any proposal.
"How can there be a consensus when two of us are saying we're not with it? It was translated into a consensus without our consent," Philippine Foreign Secretary Albert del Rosario told reporters.
The territorial conflicts have underscored a major ASEAN weak spot—the ease by which any member country can torpedo any plan through its consensual decision-making—as the bloc tackles ambitious dreams like a plan to turn the economically vibrant region of 600 million people into an E.U.-like community by 2015.
Despite its shortcomings, ASEAN has loomed as a major battleground for influence in Asia, where Obama's trip highlights an American pivot to the region following years of engagement in Iraq and Afghanistan. China, the Asian superpower, has acted to protect its home ground. ASEAN is clearly pinned in between.
A more troubling diplomatic debacle over the territorial disputes erupted in July, when Cambodia failed to publicly issue a traditional after-conference communique after a foreign ministerial meeting—an embarrassing failure that was a first in the bloc's 45-year history. Vietnam and the Philippines have insisted that the joint statement simply state that the South China Sea rifts were discussed, but Cambodia adamantly refused, echoing China's line to keep a lid on public discussions of the disputes.
Ernest Bower of the Center for Strategic & International Studies in Washington, D.C., said the imbroglio in July showed that as long as any ASEAN country remains weak and vulnerable to muscling from a major power, the entire group could be compromised.
"ASEAN learned a hard lesson from the event: namely that they should never again allow a fellow ASEAN member country to feel so isolated, exposed or dependent on any foreign power that the country feels compelled to step beyond ASEAN protocols ... in a way that damages the organization's interests and profile," Bower said.
In the ongoing summit, Indonesia proposed an emergency hot line be established by China and rival nations to allow them to communicate and rapidly end any accidental clash that could get out of hand in the troubled waters.