The Japanese Ground Self-Defense Force and the US Army have begun a joint drill aimed at boosting cooperation in cyberspace and other new areas of defense.
The annual exercise is dubbed "Yama Sakura." It started on Monday at several locations in Japan, with about 6,600 personnel taking part.
It assumes the greater Tokyo area and Japan's southwestern islands have been hit by ballistic missile and guerrilla attacks, while remote islands have been simultaneously invaded.
Commanders respond to the situation by using computers to direct troops.
The two sides will determine how they should work together beyond the conventional fields of land, sea and air. They will also practice dealing with cyber and electromagnetic wave attacks that disrupt chains of command.
Lieutenant General Takayuki Onozuka of the GSDF Eastern Army told reporters it is important that the Japan and US sides make the alliance more effective by merging their capabilities in both conventional and new domains.
Lieutenant General Gary Volsky of the US Army First Corps stressed the need for improved interoperability with the Japanese side, citing the complexity of the regional environment.