Trouble arises from within; the external enemy is the lesser worry.
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From 《论语·季氏》: 吾恐季孙之忧，不在颛臾，而在萧墙之内也 - Confucius warned that the real danger came not from a foreign state but from within the lord's own household. The set phrase 祸起萧墙 ('disaster rises from the screen wall') comes from this passage.
