The arrest of a US Marine in Okinawa on suspicion of murder has put the US-Japan Status of Forces Agreement (SOFA) back in the spotlight. Here is how such cases work.

Under SOFA, Japan has primary jurisdiction over crimes committed by US military personnel while off duty. The United States retains jurisdiction for acts done in the performance of official duty. A suspected murder and robbery in a hotel is an off-duty matter, so Japanese police investigate, arrest and prosecute, which is why the Marine is in Japanese custody.

This division of authority was designed to balance the US military's operational requirements with Japan's sovereignty over criminal justice. In practice, serious off-duty crimes by service members are handled by Japanese courts.

The history explains the sensitivity. Most of the roughly 18,000 Japan-based US Marines are concentrated in Okinawa, which the US administered until 1972. Noise, aircraft accidents and misconduct have fed decades of local resentment. The 1995 rape of a 12-year-old Okinawan girl by three servicemen triggered mass protests that still shape the debate.

Prime Minister Sanae Takaichi has demanded maximum US cooperation and stricter discipline. The case could complicate plans to move Marines from the crowded Futenma air base to Henoko.

What happens next follows the standard path: Japanese investigators build the case, prosecutors decide on indictment, and the trial proceeds in a Japanese court, with the US cooperating as pledged.