Crown Prince Dan of Yan ignored his tutor's warning twice — and lost the state

Crown Prince Dan of Yan had been a hostage in the Qin court as a young man and came home convinced Qin meant to swallow every state around it, Yan included. When the Qin general Fan Wuji fell from favor and fled to Yan for refuge, Dan's tutor Ju Wu told him not to take the man in: harboring a wanted Qin general in a small, exposed state was like throwing meat onto a hungry tiger's path. Qin's king was violent by temperament; a direct grievance against Yan was courting a war Yan could not win. Ju Wu urged sending Fan Wuji away and building alliances with other states instead.

Dan refused. Fan Wuji had come to him in desperate straits with nowhere else to go, and turning him away felt like a betrayal Dan could not live with. He kept Fan Wuji in Yan. Ju Wu's second warning followed soon after: even if the harboring could not be undone, a slow, coordinated diplomatic response was still safer than a lone strike at the Qin king. Dan rejected that too, judging Yan too weak and too short on time for alliances to work, and settled on sending a single assassin, Jing Ke, to kill King Zheng of Qin under cover of a diplomatic gift.

In 227 BC Jing Ke reached the Qin court carrying a rolled map that concealed a poisoned dagger, and Fan Wuji's own severed head as a second offering to secure an audience. He unrolled the map and lunged with the dagger. He missed. King Zheng drew his own sword, cut Jing Ke down, and survived untouched — handing Qin exactly the grievance Ju Wu had warned the harboring would create.

Qin's general Wang Jian marched on Yan immediately. The capital fell within a year, and King Xi of Yan had his own son executed in 226 BC trying to appease Qin. It did not work. The remnant of Yan held out a few more years and was annexed outright in 222 BC, among the last states to fall as Qin unified China.