The New York Times reports that while he was in Japan, on Thursday, April 24, 2014:
Obama … declared that the United States was obligated by a security treaty to protect Japan in its confrontation with China over a clump of islands [the Senkaku or Diaoyu islets] in the East China Sea. But he stopped short of siding with Japan in the dispute regarding who has sovereignty over the islands, and carefully calibrated his statement to avoid antagonizing China.
The net result, seen in a news conference in which the leaders referred to each other a bit stiffly as Barack and Shinzo, was an alliance clearly on firmer footing than it was earlier, but still vulnerable to political frailties on each side. […]
The president’s statement about the United States’ obligations toward Japan was important because it was the first time he had explicitly put the disputed islands under American protection, though Defense Secretary Chuck Hagel recently made the same statement and the policy has been held by successive administrations.
“This is a very important turning point for the United States-Japan alliance because it means the period of drift under President Obama has finally come to an end,” said Yuichi Hosoya, an expert on American-Japanese relations at Keio University in Tokyo. “The fact that this was said by the president will have a huge psychological impact on Japanese officials and people.”
The Chinese government reacted swiftly, saying it was “firmly opposed” to Mr. Obama’s position. More than anything, Mr. Obama appeared eager to defuse tensions over the islands, referring to them as a “rock” and saying they should not be allowed to derail a relationship that could otherwise be productive.
“It would be a profound mistake to continue to see escalation around this issue rather than dialogue and confidence-building measures between Japan and China,” Mr. Obama said.
[Japan’s prime minister] Mr. Abe said he was encouraged by Mr. Obama’s pledge to protect the islands. “On this point,” he said, “I fully trust President Obama.”
To South Korea:
The AP reports that while he was in Seoul, on April 26, 2014:
President Barack Obama warned North Korea on Saturday that the United States “will not hesitate to use our military might” to defend allies, as he sought to showcase U.S. power in the region amid China’s growing influence and Pyongyang’s unpredictable nuclear threats.
“The commitment that the United States of America has made to the security of the Republic of Korea only grows stronger in the face of aggression,” Obama said in a speech to some of the 28,000 American service members stationed in South Korea to keep watch on its northern neighbor. “Our alliance does not waiver with each bout of their attention seeking. It just gains the support of the rest of the world.” […]
He said real strength comes from having an open participatory democracy, open markets and a society free to speak out against its government.
“We don’t use our military might to impose these things on others, but we will not hesitate to use our military might to defend our allies and our way of life,” Obama said to cheers from the uniformed troops who filled a field house at Yongsan Garrison, headquarters for U.S. forces in South Korea.
Obama’s 10-minute speech followed a rare joint defense briefing with South Korean President Park Geun-hye that focused on efforts to counter the North’s nuclear ambitions.
See also “Pentagon official: U.S. budget will not allow an Asia pivot,” March, 2014.