Qin's young general said 200,000 could take Chu — they were routed

By 225 BCE the kingdom of Qin had swallowed Han, Zhao and Wei and was planning the war against Chu — the largest of the rival states, stretching across the Yangtze. King Zheng asked his generals how many men the campaign would need. The veteran Wang Jian answered that it could not be done with fewer than 600,000. The young general Li Xin replied that 200,000 would suffice. The king judged Wang Jian old and timid, and gave the command to Li Xin.

Li Xin and Meng Tian led the 200,000 south and won early engagements. But the Chu commander Xiang Yan refused to commit his main force. He shadowed the Qin column for three days and three nights without rest, then fell on it — smashing two camps and killing seven commanders. Lord Changping, a Qin defector, rebelled in the rear at the same moment. The Qin army collapsed and fled. It was among the worst defeats Qin suffered in its entire wars of unification.

King Zheng went in person to Wang Jian's estate at Pinyang to apologise and ask him back. Wang Jian accepted — but only on his original terms: 600,000 men, the whole mobilised strength of Qin. Given them, he entrenched at Pingyu and refused battle for months until the Chu army, bored and under-supplied, began to disperse; he then attacked its retreating columns, killed Xiang Yan, and in 223 BCE took the Chu capital of Shouchun. Chu was finished.

The case is not that Li Xin was a poor general — he later served capably under Wang Ben in the conquests of Yan and Qi. The case is the decision that preceded him: a ruler who chose the cheaper estimate over the honest one, and so paid for the campaign twice.