The author is correct that the US is bound by treaty to protect Japan, but it isn't just the US. All of NATO have a mutual defense agreement. That's why NATO is (or was, I'm not sure anymore) involved in the Afghan war . If China wants to test the US, they have all the powerful members of the EU to deal with, too. China has strength in numbers, but I don't think its a smart idea to underestimate the US Navy. Foreign Policy did an interesting assessment of the situation last year. Its worth reading. They think that even without US involvement, China stands to lose.