BEIJING >> As Barack Obama embarks on what is likely to be his final trip to Asia as president, attention is returning to what is known as the U.S. "pivot" to the continent launched during his first term.
The policy adjustment aimed to reinforce alliances and shift military assets to a region that has grown in importance alongside the rise of China as a global economic and political power. A look at some of the impacts on different countries in the Asia-Pacific region:
China: key economic partnership meets regional rivalry
The U.S. shift of focus to Asia has been driven by China's emergence as a global force and America's rival in the region. Such frictions have persisted despite an economic relationship that has seen the world's largest economies and biggest military spenders amass two-way trade of more than $600 billion in 2015.
On the military side, the U.S. pivot chiefly involves the reassignment of 60 percent of the Navy fleet to Asia, the rotation of Marines through Australia and stronger cooperation with the Philippines, mainly as a response to China's robust assertions of its claim to virtually the entire South China Sea.
China has strongly criticized the U.S. approach and pressed ahead with island-building including airstrips, harbors and other infrastructure of potential military use. The U.S. has refused to recognize the new features as islands with sovereign rights and has repeatedly sent warships and planes near them in freedom of navigation missions.
Farther north in the East China Sea, the U.S. has ignored China's declaration of an aircraft defense identification zone that encompasses islands controlled by American treaty partner Japan. South Korea's plan to deploy an American missile defense system has also aroused China's ire, sending relations between Seoul and Beijing to their lowest level in years.
China is also the largest foreign holder of U.S. debt, and by some estimates, Chinese foreign direct investment into the U.S. has started to outstrip the flow of U.S. investment into China.
Philippines: A love-hate relationship
The American pivot to Asia coincided with increasing Philippine insecurity over China's assertiveness in the disputed South China Sea.
The symbiosis sparked an upswing in relations under former President Benigno Aquino III at the same time as ties with Beijing strained after Chinese ships seized the disputed Scarborough Shoal in 2012. The antagonism worsened when Aquino's government sued China the following year before an international arbitration tribunal over contested territories.
In 2014, the long-time treaty allies signed a defense pact allowing American forces to temporarily base in designated Philippine military camps. In July this year, the tribunal invalidated China's vast claims but Beijing dismissed the ruling as a sham.
By then, Aquino has been succeeded by Rodrigo Duterte, who made known his dislike for U.S. policies and its ambassador and announced he will chart an independent foreign policy not dependent on Manila's former colonizer. While there hasn't been a sign that he will roll back defense cooperation with the U.S., Duterte has also refrained from pressing China to comply with the arbitration ruling and has cozied up publicly to Beijing.
Australia: U.S. ally within China's economic orbit
Australia and the U.S. have increased their military cooperation as part of the pivot, with American Marines now rotating through a training hub in the northern port city of Darwin. Meanwhile, an Australian army general was appointed to a senior position in the U.S. Army Pacific contingent.
At the same time, Canberra remains strongly committed to its economic relationship with China. While China's economic slowdown has reduced its demand for Australian iron ore and other commodities, Chinese companies remain keen investors in the Australian economy. Wealthy Chinese also spend heavily on Australian higher education, vacations and real estate.
The balance between economic and security interests has not always been easy to maintain. Australia has been supportive of the U.S. freedom of navigation exercises in disputed areas of the South China Sea, and has regularly sent its own air force patrols over the region, drawing anger from Beijing. Yet Australian public opinion remains strongly in favor of close ties with China.
Japan and South Korea: Key allies despite historical differences
Relations between South Korea and Japan, two key U.S. allies in Asia, have seen numerous setbacks in past decades due to bitter rows over history. It appeared the ties had sunk to one of their lowest ebbs following the 2012 inauguration of Japanese Prime Minister Shinzo Abe, who, by taking a more hawkish and nationalistic stance, raised suspicions in Seoul that he was trying to obscure Japan's brutal colonial rule of the Korean Peninsula.
The Obama administration grew increasingly frustrated about the poor relations between its allies, which together host about 80,000 U.S. troops and are critical to Washington's plans to better deal with a rising China and North Korean threats.
The U.S. worked behind the scenes to bring together Abe and South Korean President Park Geun-hye for a summit with Obama in March 2014. It resulted in the opening of talks between South Korea and Japan over resolving the issue of South Korean women who were forced into sexual slavery by Japan's military in World War II.
In October 2015, on the sidelines of a trilateral summit that also included China, Park and Abe met for the first formal talks in more than three years. South Korea and Japan two months later reached an agreement for Japan to provide 1 billion yen ($9.9 million) to a South Korean foundation established to help support former sex slaves.
Japan: Newly assertive U.S. ally in northeast Asia
Like South Korea, Japan is host to a large U.S. military presence, numbering about 50,000 troops at present, and has a formal mutual protection alliance with Washington.
There are concerns in Japan about the U.S. commitment to its defense in light of U.S. budget constraints and war fatigue after Iraq and Afghanistan. Specifically, there are questions over what the U.S. would do if China were to attempt to seize control of uninhabited islands in the East China Sea that both countries claim.
Obama and other senior U.S. officials have tamped down those concerns somewhat by publicly reiterating that the U.S. is treaty-bound to defend Japan.
Despite China's strong objections, the government of Prime Minister Shinzo Abe has also pushed through contentious legislation to loosen post-World War II restrictions on the Japanese military to allow Japan to contribute more to regional defense. Japan has also deepened its security ties with Australia and Southeast Asia.