He quoted 'even an army can lose its commander' to urge his teammates to hold on.
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history
From 《论语·子罕》 (Analects, Book IX). Confucius said: 三军可夺帅也,匹夫不可夺志也 — 'The commander of three armies may be taken captive, but the will of even a common man cannot be wrested away.' A classic statement of inner resolve.
usage
Almost always quoted with its second half 匹夫不可夺志也 — the first half only sets up the contrast. 夺 here means 'to wrest away / to take by force,' not just 'seize.'