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During the Spring and Autumn period, various feudal states were constantly at war. The state of Jin, allied with Lu, launched an attack on Zheng.
Seeing their formidable might, Zheng had no choice but to send Prince Ma Fei as their representative to Xi, where he entered into a blood oath alliance with Jin's general, Xun Yan, signing a treaty.
Not long after, the state of Chu sent troops to attack Zheng. However, Jin stood by and offered no assistance.
Prince Ma Fei believed that Jin, before the blood from their oath had even dried, would not come to their aid, and thus formed an alliance with Chu instead.

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Open the tool →口血未干 (kǒu xuè wèi gān) is a Chinese idiom (chengyu) meaning “to break an oath, promise, or agreement very soon after it has been made; to go back on one's word almost immediately”.
口血未干 is pronounced kǒu xuè wèi gān in Mandarin Chinese.
Literally, 口血未干 means “the blood from the mouth is not yet dry”. It is used to mean “to break an oath, promise, or agreement very soon after it has been made; to go back on one's word almost immediately”.
口血未干 comes from a classical Chinese story dating back to ancient China. The full story is on this page.
古时订立盟约,要在嘴上涂上牲口的血。指订约不久就毁约。