[7] Sun quickly deserted to the National Revolutionary Army during the Northern Expedition, however, and would repeatedly change sides in the following years.

[11] In 1930, he fought with warlords Feng Yuxiang and Yan Xishan[4] in the Central Plains War against Chiang Kai-shek's government.

[12] As the conflict turned against the warlord alliance, Sun's army increasingly suffered from mass desertion, so that he "desperately" launched recruitment campaigns in northern Henan and southern Hubei while the fighting was still ongoing.

[13] Realising that the Central Plains War was lost, Sun eventually retreated with his remaining army from the front at Bozhou, and joined the Nationalist-allied forces of Zhang Xueliang.

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.

[16] As the entire Chinese defense of Rehe 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.

[17] The national government feared Sun Dianying would cooperate with Feng's Anti-Japanese Army, allowing them to use the railroad to support their forces.

In March, Sun's army was defeated by the Ma forces, while his route of retreat was cut off by Yan Xishan and troops loyal to Chiang Kai-Shek.

He consequently surrendered in April, and went to Taiyuan to live in seclusion, while the remnants of his defeated troops were incorporated into Yan Xishan's provincial forces.

[citation needed] He was eventually appointed as general of the NRA's 5th Army, but alongside his superior Pang Bingxun defected to the Japanese in 1943.