US President Barack Obama's attendance at an annual summit of Southeast Asian leaders on Tuesday thrust him right in the eye of the region's most stormy dispute: the long-raging rivalry between China and five neighbours for control of strategic and resource-rich waters in the South China
The inability to resolve these territorial conflicts has become a major impediment to the 10-member ASEAN as it tackles dreams like a plan to turn the economically vibrant region of 600 million people into an EU-like community by 2015.
Neither the US nor China is a member of ASEAN, but each has strong supporters in the group. Summit host Cambodia, an ally of China, has tried at this week's summit to shift the focus to economic concerns, but Beijing's territorial disputes with its ASEAN neighbours - including staunch US ally the Philippines - have yet again overshadowed discussions.
The disagreement sparked a tense moment Monday when Philippine President Benigno Aquino III challenged Cambodian PM Hun Sen, who had tried to cut off a discussion of the territorial disputes.
Into this heated atmosphere came Obama, who flew to Phnom Penh for Tuesday's expanded East Asia Summit, in which the 10 ASEAN countries were joined by eight other nations, including China and the US.
Behind closed doors, the Chinese and Philippine leaders pressed their territorial claims while others called for restraint. After the summit, the exchange shifted to the lobby, where diplomats of the two countries reiterated their positions.
The dispute, and Obama's presence, highlights how ASEAN has become a battleground for influence in Asia, just like the South China Sea.
The US is pushing its "Pacific pivot" to the region following years of engagement in Iraq and Afghanistan. China, the Asian superpower, has acted to protect its home ground.
Southeast Asia is pinned in between, and the lack of consensus over the maritime disputes has pushed the bloc's other work to the sidelines.
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