塘沽協定
In early 1933, the Imperial Japanese Army launched Operation Nekka to conquer Jehol Province. At the time, Sun commanded the 41st Army of 30,000 men, most of them garrisoned in Lingyuan; one of the soldiers under his command during this conflict was writer Duanmu Hongliang. As the Japanese closed in on Lingyuan, Sun did not resist, as he and his sub-commanders had been bribed by the invaders. They allowed the Japanese to occupy the city almost without resistance. Only a few units under Sun's command refused to retreat from Lingyuan and fought the Japanese to the death, while he moved his army to Chahar Province. As the entire Chinese defense of Jehol collapsed, some of Sun's units lost contact to their headquarters and were left behind; one of his regiments consequently launched a counter-attack on Chifeng, not knowing that other NRA forces had already fled the province. As the Chinese high command initially refused to admit that Sun had been bribed, and some troops under his command had valiantly fought the Japanese, Sun's reputation was slightly rehabilitated in course of the campaign, an opportunity he seized to expand his strength. At the time of the Tanggu Truce, Sun's troops garrisoned the strategic Peiking–Suiyuan Railway.