In Burma the British, under intense pressure, made a fighting retreat from Rangoon to the Indo-Burmese border. This cut the Burma Road which was the western Allies' supply line to the Chinese Nationalists. In March 1942, Chinese Expeditionary Force started to attack Japanese forces in northern Burma. On 16 April, 7,000 British soldiers were encircled by the Japanese 33rd Division during the Battle of Yenangyaung and rescued by the Chinese 38th Division led by Sun Li-jen. Cooperation between the Chinese Nationalists and the Communists had waned from its zenith at the Battle of Wuhan, and the relationship between the two had gone sour as both attempted to expand their area of operations in occupied territories. Most of the Nationalist guerrilla areas were eventually overtaken by the Communists. On the other hand, some Nationalist units were deployed to blockade the Communists and not the Japanese. Furthermore, many of the forces of the Chinese Nationalists were warlords allied to Chiang Kai-Shek, but not directly under his command. "Of the 1,200,000 troops under Chiang's control, only 650,000 were directly controlled by his generals, and another 550,000 controlled by warlords who claimed loyalty to his government; the strongest force was the Szechuan army of 320,000 men. The defeat of this army would do much to end Chiang's power." The Japanese exploited this lack of unity to press ahead in their offensives.