On 1 March 1951 Mao sent a cable to Stalin, in which he emphasized the difficulties faced by Chinese forces and the urgent need for air cover, especially over supply lines. Apparently impressed by the Chinese war effort, Stalin finally agreed to supply two air force divisions, three anti-aircraft divisions, and six thousand trucks. PVA troops in Korea continued to suffer severe logistical problems throughout the war. In late April Peng Dehuai sent his deputy, Hong Xuezhi, to brief Zhou Enlai in Beijing. What Chinese soldiers feared, Hong said, was not the enemy, but that they had nothing to eat, no bullets to shoot, and no trucks to transport them to the rear when they were wounded. Zhou attempted to respond to the PVA's logistical concerns by increasing Chinese production and improving methods of supply, but these efforts were never completely sufficient. At the same time, large-scale air defense training programs were carried out, and the Chinese Air Force began to participate in the war from September 1951 onward.