Thursday's agreement between the United States and Japan to revise defense cooperation guidelines touched on a number of core issues, such as transferring U.S. Marines to Guam, building a contentious replacement in Okinawa for the Futenma base, and cooperating on ballistic missile defense and cybersecurity.

But the meeting between Foreign Minister Fumio Kishida, Defense Minister Itsunori Onodera, U.S. Secretary of State John Kerry and U.S. Defense Secretary Chuck Hagel avoided what critics of the bilateral alliance say is the most taboo subject of all — revising the Status of Forces Agreement.

Politicians on the right and left of the political spectrum have long sought to change the agreement, which has never been formally revised since coming into force in 1960.