This book focuses on the British Commonwealth armies in SE Asia and the SW Pacific during the Second World War, which, following the disastrous Malayan and Burma campaigns, had to hurriedly re-train, re-equip and re-organise their demoralised troops to fight a conventional jungle war against the Imperial Japanese Army (IJA). British, Indian and Australian troops faced formidable problems conducting operations across inaccessible, rugged and jungle-covered mountains on the borders of Burma, in New Guinea and on the islands of the SW Pacific. Yet within a remarkably short time they adapted to the exigencies of conventional jungle warfare and later inflicted shattering defeats on the Japanese. This study will trace how the military effectiveness of the Australian Army and the last great imperial British Army in SE Asia was so dramatically transformed, with particular attention to the two key factors of tactical doctrine and specialised training in jungle warfare. It will closely examine how lessons were learnt and passed on between the British, Indian and Australian armies. The book will also briefly cover the various changes in military organisation, medical support and equipment introduced by the military authorities in SE Asia and Australia, as well as covering the techniques evolved to deliver effective air support to ground troops. To demonstrate the importance of these changes, the battlefield performance of imperial troops in such contrasting operations as the First Arakan Campaign, fighting along the Kokoda Trail and the defeat of the IJA at Imphal and Kohima will be described in detail.