'Bu' was a Japanese plan by Lieutenant General Renya Mutaguchi’s 15^th Army for a limited offensive in north-western Burma to cross the Chindwin river at points to the south of Homalin and then proceed to the seizure of the hills between the Yu and Chindwin rivers as a protective barrier for the Japanese in the light of experience with 'Longcloth' (1^st Chindit Expedition) during February and March 1943 (May 1943).

Mutaguchi anticipated that such a move could be completed before the onset of the 1943 monsoon to provide protection and also, according to his own longer-term planning, to create the springboard for the 'U' major offensive he was planning toward north-eastern India for the dry season of 1944.