Yunnan was one of, if not the, most important parts of China from the perspective of the Nationalists during the Sino-Japanese War. The province was a major resource base, military and air base, and provided a safe haven for China's government and nascent industry, largely safe even from Japanese bombing.

The province was also, unlike the capital at Chongqing, connected by rail to Japanese-occupied Vietnam, and elsewhere in China Japanese advances tended to be anchored on rail and river lines for supply

The province is highly mountainous and heavily forested - but that did not stop the Japanese from trying to invade Yunnan from the West, which has even worse terrain, was close to Allied military formations and air support, and lacked rail connections the Japanese could use

So in summation - Eastern and Central Yunnan was probably the second most important part of China for the Nationalist war effort, and from 1940 onward Japan had a border with the province, with a rail line leading directly to the province's capital city and heart. Despite this Japan never tried to invade the province from this direction, only attacking Yunnan from its far less hospitable West, where both Chinese and Commonwealth strength was

Why?