The Japanese Fifteenth Army commander, General Iida Shojiro, now turned his 33rd and 55th Divisions north. His objective was to seize the oil fields and refinery at Yenangyaung on the Irrawaddy, north of Prome. Supported by Burmese dissidents in a growing unconventional warfare capability, the Japanese employed frontal assaults, small-unit infiltrations, guerrilla operations, and sabotage to keep Allied forces on the defensive. Lieutenant William Slim's I Burma Corps held a defensive line with the 17th Indian Division forward of Prome on the Irrawaddy River and the 1st Burma Division near Toungoo and the Sittang River. Three Chinese armies—the Fifth, Sixth, and Sixty-Sixth—were moving south to reinforce. U.S. Lieutenant General Joseph Stilwell, who arrived in China in March to lead the U.S. effort to train the Nationalist Chinese Army, planned to reopen the land supply routes to China.
The Japanese captured Prome and Toungoo in early April. The British then concentrated their resources near Allanmyo, on the Irrawaddy, redeploying the 17th and 1st Divisions from Toungoo. Slim's mission was to prevent the Japanese from taking the oil fields and to defend northern Burma. Critical to this effort was the ability of the Chinese to hold in the east in the Sittang Valley and the speed with which they could bring in reinforcements to allow the British to redeploy and form a reserve force large enough to conduct large-scale offensive operations to stop the Japanese advance up the Irrawaddy. On 8 April, Slim moved the I Burma Corps headquarters from Taungdwingyi to Magwe, just south of Yenangyaung. The oil fields were still producing fuel for the Allied forces, but as a precaution, they were to be destroyed rather than having them seized.
The main Japanese effort against Yenangyaung began in earnest on 10 April with direct assaults against forward British brigades and attempts to infiltrate small elements and units disguised as Burmese forces and Chinese. By 14 April, the Japanese had split the defending British forces. The next day, Slim ordered the destruction of the oil fields and refinery as heavy fighting continued in the Yenangyaung area.
British forces, now supported by the 38th Chinese Division, continued to engage the Japanese 33rd Division. Slim prepared an offensive thrust in which the 38th Division would reduce pressure on the 1st Burma Division, allowing elements of the 17th Indian Division to counterattack the Japanese. As the Battle of Yenangyuang continued, however, the collapse of Chinese forces in the Sittang area put the entire British effort in the west at risk. At the same time, the Japanese cut off the 1st Burma Division, but British counterattacks, coupled with attacks by the Chinese 38th Division, allowed Slim to extract the 1st Burma Division. By 25 April, however, Allied forces had begun a general withdrawal. The complete loss of Burma and the retreat of all British forces to India was now only a matter of time. J. G. D. Babb
Allen, Louis. Burma: The Longest War, 1941–1945. New York: St. Martin's Press, 1984.; Romanus, Charles F., and Riley Sunderland. China-Burma-India Theater: Stilwell's Command Problems. Washington, DC: Center of Military History, 1985.; Slim, William J. Defeat into Victory. London: Macmillan, 1986.
J. G. D. Babb