As President Barack Obama attends a key trade forum in Indonesia this weekend, the U.S. is moving beyond its decadelong focus on the Middle East to concentrate again on the powerhouse economies of East Asia at a time when the biggest success story of them all—China's rise to become the world's second-largest economy—is transforming the entire Asia-Pacific region.

Once America's fierce war-time adversary, Vietnam is now emerging as one of Washington's most important new allies in providing a diplomatic and commercial counterpoint to China's growing clout, and Washington's warming relationship with Hanoi perhaps best illustrates America's full-court press in the region across a broad spectrum of fronts.

In a notable reversal from the war years, a U.S. Navy vessel called in at Vietnam's Cam Ranh Bay naval base in August for the first time in more than three decades. The USNS Richard E. Byrd spent seven days undergoing repairs at the shipyards there, which have been occupied by French, American, Soviet and, finally, Vietnamese forces over the course of 60 years. The visit added to strengthening ties between the two countries' navies, after the U.S. and Vietnam also held joint noncombat-training exercises in July.

Cam Ranh Bay was one of the U.S.'s largest military bases during the Vietnam war, and the scheduled repair stop came close on the heels of American statements that the busy shipping lanes of the South China Sea should be left free—a clear warning to China's claims to sovereignty over the entire region, and a move which infuriated Beijing and its increasingly assertive naval forces.

Military strategists describe the site as hugely attractive—a deep-water bay offering protection from the frequent typhoons that batter the region and which also provides a strategic base on the South China Sea. Vietnam's leaders are refurbishing the tightly-guarded facility in a bid to encourage other countries' vessels to regularly stop by for repairs or military exercises, effectively internationalizing shipping lanes nearby and counterbalancing China's reach in the region.

It's also a useful way for the U.S. to forge stronger ties. One of Washington's top diplomats, Assistant Secretary of State Kurt Campbell, argues that a stronger American presence could help secure the economic growth of all countries in region, including China, by ensuring stability and preventing small regional spats from escalating into more disruptive conflicts.

"I would argue that in many respects the last 30 or 40 or 50 years in which we've seen remarkable progress economically and politically in many ways have been underwritten by the full presence of the United States, and we will seek to continue that effort and to diversify it," he said in Bangkok last month.

The U.S. is also tapping into growing environmental concerns in the region—much of it prompted by China's massive need for resources. In the rice-farming delta of the Mekong River, for instance, Americans are fanning out to help Vietnam cope with the potential environmental impact of China's dam-building projects further upstream. Among them are geologists from the U.S. Geographical Survey who are helping map and counteract some of the damage caused by China's thirst for hydroelectric power. The dams prevent nutrient-rich silt flowing down the delta, reducing land levels in the Mekong delta and allowing salt water to flow up the Mekong as far as 60 miles inland, destroying valuable farmland.

"Once you start mucking about with a river, it can cause all sorts of problems," says Richard Cronin, a Vietnam War veteran and an expert on the environmental challenges facing the Mekong river system who now heads the Southeast Asia program at the Washington-based Stimson Center.

Indeed, China's dam program is triggering a growing environmentalist movement here that's often colored by strong anti-Chinese sentiments. In Myanmar, too, anti-Chinese environmentalists recently pressured the military-backed government to suspend construction on a massive new hydroelectric complex that would have flooded a large chunk of the country in order to feed China's growing appetite for power.

"The U.S. won't admit as much, but they are partly tapping into this anti-Chinese feeling," says Ian Storey, a regional security expert at the Institute of Southeast Asian Studies in Singapore.

Vietnam is also one of the players in the U.S.'s plans for a new free-trade group known as the Trans-Pacific Partnership, which also includes countries such as Chile, Malaysia and New Zealand. The U.S. already is a major investor in Vietnam, spearheaded by Intel Corp.'s $1 billion investment in a chip plant in Ho Chi Minh City.

Officials in Washington privately say they hope the trade plan will overtake China's proposals to create an Asia-only trade bloc and help dilute Beijing's commercial and military influence in a region it increasingly considers its own backyard.

So far, the trade proposal seems to be working. Japan, Mexico and Canada have said they want to join, too. Even China's President Hu Jintao acknowledged that Washington's proposal on a cross-Pacific pact could be one path to a broader trade deal—something some U.S. officials already privately welcome as a way of keeping China engaged even as America retreads its path back into Asia.

By James Hookway - The Wall Street Journal - November 17, 2011