After Cao Cao defeated Yuan Shao at Guandu, he took Yuan Shao’s papers. Among them he found letters that men in the capital and in his own army had written to Yuan Shao. He burned them all.
The Records of the Three Kingdoms records this. A commentary quotes Cao Cao’s reason: “When Yuan Shao was strong, even I could not be sure of keeping myself safe. How much less could the others?”