Aspects of this topic are discussed in the following places at Britannica.
...Wuzhou in far northern Shanxi, who had been a constant threat since 619, was finally defeated and killed by his former Turkish allies in 622. In the south during the confusion at the end of the Sui, Xiao Xian had set himself up as emperor of Liang, controlling the central Yangtze region, Jiangxi, Guangdong, and Annam (Vietnam). The Tang army descended the Yangtze from Sichuan with a great fleet...
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