In the next few years Liu Yongfu established a profitable protection racket on commerce on the Red River between Son Tay and Lao Cai. Traders were taxed at the rate of 10% of the value of their goods. The profits that accrued from this extortion were so great that Liu's army swelled in numbers during the 1870s, attracting to its ranks adventurers from all over the world. Although most of the soldiers were Chinese, many of the junior officers were Americans or European soldiers of fortune, some of whom had seen action in the Taiping Rebellion, and Liu used their expertise to transform the Black Flag Army into a formidable fighting force. Liu commanded 7,000 black flag soldiers from Guangdong and Guangxi around Tonkin.