Campaign against Xiao Xi
In fall 621, Emperor Gaozu commissioned Li Xiaogong, with Li Jing as his assistant, to launch a major attack on Xiao Xi's Liang state, with a cousin of Li Xiaogong's, Li Yuan the Prince of Lujiang and the other generals Tian Shikang and Zhou Faming attacking on other fronts. At that time, the Yangtze River had a high water level and was flowing rapidly. While many officers under him suggested delaying the campaign, Li Xiaogong, perhaps with Li Jing's suggestion, decided that indeed, the rapid water was a good opportunity to launch a surprise attack on Xiao Xi's capital Jiangling . After defeating the Liang general Wen Shihong , he put Jiangling under siege, cutting off Xiao Xi's communications with the armies in the rest of his territory. He, again possibly with Li Jing's suggestion, floated the Liang ships that he captured down the Yangtze, to confuse the approaching Liang relief forces into believing that Jiangling had fallen already. Xiao, not knowing that the relief forces were only a few days away, surrendered to Li Xiaogong. By the suggestion of Xiao's official Cen Wenben, who had persuaded Xiao to surrender, Li Xiaogong strictly prohibited pillaging and retribution against Liang generals. The Liang provinces, upon hearing the news of Jiangling's fall, soon largely submitted to Tang. Li Xiaogong delivered Xiao to Chang'an, where Emperor Gaozu executed him.
He drank the blood-water with ease, impressing and calming his generals. He then sailed down the Yangtze, attacking and defeating the Song generals at multiple battles. Fu ordered his generals Feng Huiliang and Chen Dangshi to take up position at Mount Bowang , preparing for the assault, instructing Feng and Chen not to engage Li Xiaogong, but to wear him down. However, Li Xiaogong cut off their supply route, and when Feng and Chen's food supplies ran low, they challenged Li Xiaogong. Against suggestions by some officers that he bypass Feng and Chen and attack the Song capital Danyang directly, Li Xiaogong, perhaps with Li Jing's suggestion, confronted Feng and Chen, initially using the weaker segment of his forces to battle Feng and Chen and, after initial losses, draw Feng and Chen in deeper, and then attacked them, defeating them decisively, with the aid of Li Fuwei's subordinate general Kan Leng . After the victory, Li Xiaogong and Li Jing attacked Danyang. Fu, in fear, abandoned Danyang and fled east, but was captured by the local men and delivered to Danyang.
Li Xiaogong was one of the few Tang generals who achieved great independent success without being associated with Emperor Gaozu's talented son , and he enjoyed great reputation among the soldiers. As he was put in charge of the lower Yangtze region after defeating Fu, he rebuilt the important Southern Dynasties fortress of Shitou , which led to accusations that he was planning treason himself. Emperor Gaozu recalled him to the capital and had him interrogated, but after insufficient evidence was shown against him, released him and made him the minister of imperial clan affairs -- a high level position that however had relatively little power. He later successively served as the commandant at Liang Prefecture and prefect of Jin Prefecture . After Li Shimin became emperor in 626 , his title was changed to Prince of Hejian, and he was made the minister of ceremonies.