The siege was noted for the Tang army's determination to fight to the last man, as well as the large-scale cannibalism practised by the defenders, who in this way were able to hold out longer.

By the end of 756, the rebel Yan army had captured most of northern China, which then included both Tang capitals, Chang'an and Luoyang, and was home to the majority of the empire's population.

The city, therefore, formed a major obstacle for the rebels on the route from the capitals to the southeastern coast, the breadbasket of the Tang dynasty.

The administrator of Suiyang Prefecture at the time, Xu Yuan [zh] (許遠), requested help from garrisons in neighbouring cities.

At the time, Zhang Xun, formerly a county magistrate serving in the Tang government, was the leader of volunteer defenders in Yongqiu.

Recognizing the strategic importance of Suiyang, he led 3,000 men to aid its defence, bringing the total number of defenders to 6,800.

Meanwhile, Yin Ziqi mustered a huge army (estimated at 130,000 men) and started besieging the city in late January.

Zhang Xun noticed where the soldiers ran and ordered his best archer, Nan Jiyun, to shoot at Yin Ziqi.

When the inhabitants of the besieged city ran out of provisions, they started to eat horses first and then "the aged, children and women".

[4][5] According to the Old Book of Tang, Zhang Xun killed his own concubine as food for the hungry soldiers to convince them that such extreme sacrifices were worth it.

Xun fetched his concubine, killed her in front of the army and presented her to the soldiers saying: "Brothers, for the sake of your country you have defended this city with united efforts....

According to the historian David A. Graff, these numbers are "open to question" because, while there might have been 60,000 people in the city at the start of the siege, any food supplies "would have gone to the combatants on a priority basis", hence many civilians "would presumably already have died of starvation or fallen victim to 'unofficial' cannibalism by the time that the garrison began to eat human flesh".

[12] While cannibalism during sieges and famines was not unusual, this case was nevertheless "noteworthy" not only because of its apparently considerable scale, but also because it was "an organized and systematic logistical operation carried out by the soldiers of the garrison" under Zhang Xun's command, as Graff notes.

According to an exchange in the Zizhi Tongjian, Yin Ziqi asked Zhang Xun, "I heard that every time you fight, your eyes are ripped open, and your teeth are cracked.

Fearing further danger from his captives, Yin had all three men executed, along with 33 other loyal elite soldiers, including Lei Wanchun and Yao Kun.

After the war, the imperial government and literati increasingly portrayed Zhang Xun and Xu Yuan as icons of loyalty and patriotism.

The plan by the Tang court to posthumously award Zhang Xun was initially met with controversy due to the mass cannibalism in the siege.

Some court officials condemned Zhang's conduct, maintaining "that it would have been better for him to have evacuated Sui-yang than to have eaten the people entrusted to his care".

Only the arguments of one of them – Li Han, who seems to have been a close friend of Zhang Xun – are still known,[15][16] since parts of them were incorporated into the New Book of Tang[6] and the Zizhi Tongjian.

[8] Li offered three arguments in defence of Zhang's acts: Firstly, that human flesh had only been eaten as "a desperate expedient, a last resort", when other Tang forces failed to come to the rescue.

Secondly, that "his stubborn and prolonged defense had kept the rebels out of the Huai River valley and the Lower Yangtze region", preventing them from making further progress and thus greatly contributing to their ultimate downfall.

He therefore stated that even if Zhang had intended "from the very start to engage in cannibalism ..., his military accomplishment was so great that the merit and the fault would still have cancelled each other out".

Finally, Li argued that Zhang had been loyal to the government up to the death and that praise for him would "encourage others to behave in the same exemplary fashion".

[17] The arguments in favour of the defenders prevailed and Zhang was added to the list of "loyal martyrs" who were rewarded posthumous offices by the emperor.

[18] Due to the decision that their merit in maintaining the Tang victory outweighed any concerns, shrines were constructed in honour of Zhang and Xu, first in Suiyang and later also at the Lingyan Pavilion in Chang'an.

In a perilous situation, where people sometimes had to "choose between their loyalty to the dynasty on the one hand and the safety of themselves and their loved ones on the other, the court and its supporters could not afford to allow any doubt as to which considerations should be primary and which secondary".

By demonstrating so clearly that "loyalty to the dynasty" was his utmost concern, his "transgressions ... provided a particularly clear-cut object lesson in the proper prioritization of values".

[20] Tales of the heroism of the defenders were embellished both in the works of famous writers and poets during the Tang–Song period, such as Gao Shi, Han Yu, Liu Zongyuan, Wang Anshi, Sima Guang, Ouyang Xiu and Huang Tingjian, and in official histories like the New Book of Tang.

[6][7] A popular poem by the late Song politician Wen Tianxiang cited the stories of Zhang Xun and Xu Yuan as examples of loyalty and persistence to inspire resistance in the face of the Mongol invasion.