This mainly has to do with China’s clear effort to dominate the Western Pacific. Japan, no less than China, is vulnerable to disruption of important sea lanes, but China has taken the stand that it and it alone ought to be the monitors. China has claimed several China Sea islands, including a group that belongs to Japan.
The conservative leader has already taken steps to pump up Japan’s military profile. He plans to conduct mine sweeping operations in the South China Sea, consideration of defense alliances with its neighbors and passage of the biggest military budget since World War II.
Japan has also concluded that US military policy, which seeks to rely more on like-minded regional powers as partners, requires an active military. This has prompted Abe to gradually jettison Japan’s post-war pacifism.
Of course, any time Japan complains about Chinese military moves, Beijing throws World War II in Japan’s face, and this is the context of Abe’s not-very-apologetic stand.
Japan’s emperor softened Abe’s hardline my making a more fulsome apology. Foreigners, at least, took this to be a rebuke. I looked at it as a division of labor. The emperor speaks for history, Abe speaks for the Japan and its needs as he sees them now.
Stars and Stripes on Japan and changing US defense posture.
Carnegie Endowment on China, and the US-Japan alliance.