After the Marco Polo Bridge Incident, Yu, like many other intellectuals, heeded the call to serve his country through military service. Pen in hand, he entered the fray. Chiang Ching-kuo, then political director of the China Youth Corps, raised him to the post of political director of the 203rd Division.
盧溝橋事變後，余先生決定投筆從戎、響應知識分子從軍報國之號召，並被時任青年軍政治部主任的蔣經國先生拔擢為二○三師的政治部主任。