A Self-Defense Forces unit dubbed "Japan's Marine Corps" made headlines this week when details were revealed of an alleged secret 2015 pact with the U.S. over stationing the amphibious troops at Camp Schwab in Okinawa's Henoko District.

Tokyo strongly denies the report, but that allegation and China's growing maritime assertiveness have cast a spotlight on the Amphibious Rapid Deployment Brigade, the first responders in any future conflict surrounding the nation's outer islands.

What is the brigade and what is it for?