When Qin had annexed Zhao, it pressed northward against Yan. Hearing of this, the King of Yan dispatched an envoy to offer congratulations to the King of Qin. As the envoy passed through Zhao, the King of Zhao detained him. The envoy said, "Qin has swallowed up Zhao, and the King of Yan sent me to offer congratulations. The King of Qin has said nothing, yet the King of Zhao detains me—this is to cause one who comes bearing felicitations to be put to death instead. Moreover, I have heard that Qin aspires to unite the whole world under one rule; Zhao is not its only target—Yan too lies within its designs. Now that Zhao has fallen, Yan stands in peril. Yan's congratulations to Qin are not merely for Qin's sake, but also for its own security. It would be better for the King of Zhao to release me, so that one who congratulates Qin may return home. Then Yan will surely be grateful to Zhao, and Zhao may thereby secure its own position." The King of Zhao then set him free.