army said to number nearly 800,000 men (in fact 60,000 men) under the command of General Xiao Sunning swarmed out of Liao from the Naewon-song Fortress and surged across the Yalu River into Goryeo. Waves of Khitan warriors swept across the river and fanned out over the countryside. In bloody back-and-forth warfare, the fierce resistance of Goryeo soldiers at first slowed, then considerably hampered the Khitan advance at the city of Pongsan-gun. But Goryeo's army never surrendered. It stood firm against frontal attacks, broke to retreat and lay ambushes, and launched flanking attacks against the Khitan. Goryeo warriors finally halted Xiao Sunning's army at the Cheongcheon River. In the face of such quick and determined resistance, the Khitan decided that further attempts to conquer the entire peninsula would be far too costly, and sought instead to negotiate a settlement with Goryeo. Without a hint of contrition or humility, the Khitan General Xiao Sunning demanded the surrender of the former territory of Balhae to Emperor Shengzong. He asked that Goryeo sever its relations with Song dynasty and, in the boldest demand of all, that Seongjong accept vassal status under the Liao emperor and pay a set annual tribute to the Liao state. Instead of and rejecting General Xiao's demands outright, the royal court at Kaesong began a heated debate about the Khitan ultimatum. Government officials believed that acceding to General Xiao would prevent further Khitan incursions and urged the court to appease the Liao emperor. Many of the senior military commanders who had recently faced the Khitan army on the battlefield opposed accepting General Xiao’s terms, including Official Seo Hui, commander of an army group north of Anju. While the bureaucrats argued in Kaesong, General Xiao launched a sudden attack across the Cheongcheon River, directly on the Goryeo army headquarters in Anju. The Khitan assault was quickly repulsed, but it agitated the royal court to a state of near panic. In an effort to calm the court nobility, minister Seo Hui volunteered to negotiate directly with General Xiao. Both parties knew that a key factor influencing the negotiations was the heavy pressure being exerted on the Liao state by Song dynasty. In face-to-face talks with his Khitan counterpart, minister Seo bluntly told General Xiao that the Khitan had no basis for claims to former Balhae territory. Since the Goryeo dynasty was, without question, successor to the former Goguryeo kingdom, that land