After the major defense cuts that have and are taking place in the Obama years, Mr. President took advantage of his current trip to Japan to irritate and threaten the Chinese.
Obama says the post-WWII treaties with Japan obligate the United States to protect the nation of Japan if it gets in a fight. The Chinese and Japanese are in a war of words about the Senkaku Islands, which Japan has and China wants.
China, charter member of the "I Hate Japan" Club, recently declared an Air Defense Interception Zone which includes the Senkaku Islands. The Japanese, charter members of the "Let's Have a War with China" club, pronounced themselves officially not amused by the Chinese insult to Japan's national honor.
And Obama announces that he sees a treaty obligation to defend Japan.
Of course, the signatories to the NATO alliance (Poland, Estonia, Latvia and Lithuania) might caution Japan that Obama's promise to defend you is not worth a Ukrainian penny.
Of course, the fact that China has about 1.3 billion people and the U.S. has about 300 million should never influence a president to think carefully about starting a war with a country 4 times your size.