With diplomatic relations strained, China and Japan face uncertainty in their rare earth trade. China previously used export controls on these critical elements during trade disputes.

When China broke out one of its “big guns” in last year’s trade war with the US – an array of export controls on rare earth elements – it helped spur a temporary truce in the pitched conflict between the two economic superpowers. After Beijing’s announcement, many around the world expressed shock at the size and scope of China’s response to Washington’s sky-high tariffs. But for Japan, a squeeze on rare earth shipments was not so novel a concept. In 2010, following the collision of a Chinese...