The current Japanese government under the ultra right-wing nationalist Prime Minister Abe is belligerently seeking to revise the country's pacifist constitution in order to expand Japan's military presence beyond their homeland (under the euphemistic "Collective Self-Defense" initiative).
Given the country's utter lack of acknowledgement & reconciliation of its past wartime crimes against humanity during the 1st half of the 20th century, allowing Japan to initiate first military strike & deployment of its forces overseas may open the Pandora Box in East Asia.
Abe's recent visit to the Yaskuni Shrine (where 14 convicted war criminals responsible for the deaths of millions are honored) is just one of many indications that Japan is not ready to be a respectable U.S. ally for peace in Asia.