The idiom '马首是瞻' (mǎ shǒu shì zhān) originated from 'Zuo Zhuan' (Commentary of Zuo), Duke Xiang's 14th Year. Its literal meaning is 'to look to the horse's head'.

The story goes that during the Spring and Autumn Period, Duke Ping of Jin allied with twelve vassal states to attack the state of Qin. The commander of this allied army was Xun Yan.

Xun Yan initially believed that the Qin army would be overwhelmed and panic. However, Duke Jing of Qin, aware of the disunity within the allied forces, remained resolute and showed no intention of seeking peace. Faced with this, Xun Yan decided to prepare for battle and issued a strict command to all his generals: 'Tomorrow morning, at cock-crow, harness your chariots. Fill in the wells and demolish the stoves. During the battle, the entire army must look to the direction of my horse's head to determine their movements. Wherever my horse goes, you must follow.'

Unexpectedly, the commander of the lower army found Xun Yan's order too overbearing. He reacted with disdain, saying, 'The state of Jin has never issued such an authoritarian command. Why should we obey him? If his horse's head points west, I will deliberately go east.' His deputy, siding with him, also led his troops eastward. This caused immediate chaos and disarray throughout the allied army.

Witnessing the insubordination and the resulting confusion, Xun Yan, having lost control of the lower army, sighed to the sky, 'If my commands cannot be enforced, there is no hope of victory. Any engagement will only benefit the Qin army.' He had no choice but to order a full retreat for the entire army.

The idiom '马首是瞻' thus came to signify obeying orders or willingly following someone's lead.