In 1121–23, Song forces fared badly against the Liao, but the Jin succeeded in driving the Liao to Central Asia. Through the campaign, the Jurchens discovered weaknesses about the Song military based in the north (as the Chinese for so long had been sending tribute to the Liao instead of actually fighting them). Song forces had failed to make a joint attack in a siege with the Jurchens, who viewed the Song generals as incompetent. Banking on the possibility that the Song were weak enough to be destroyed, the Jurchens made a sudden and unprovoked attack against the Song in the north. Soon enough, even the capital at Kaifeng was under siege by Jin forces, only staved off when an enormous bribe was handed over to them. There was also an effective use of Song Chinese war machines in the defense of Kaifeng in 1126, as it was recorded that 500 catapults hurling debris were used. During the siege of Taiyuan, the Jin employed 30 catapults and over fifty carts protected by rawhide and sheets of iron plating so that Jin troops could be ferried to the walls safely to fill in Taiyuan city's defensive moat. The eunuch general Tong Guan, who had initially urged for an alliance with the Jurchens, was blamed for causing the war. He was eventually executed by Emperor Qinzong of Song (r. 1126–1127) after Huizong abdicated the throne to him.