The Vietnamese government, unable to confront Rivière with its own ramshackle army, enlisted the help of Liu Yongfu, whose well-trained and seasoned Black Flag soldiers were to prove a thorn in the side of the French. The Vietnamese also bid for Chinese support. Vietnam had long been a vassal state of China, and China agreed to arm and support the Black Flags and to covertly oppose French operations in Tonkin. The Qing court also sent a strong signal to the French that China would not allow Tonkin to fall under French control. In the summer of 1882 troops of the Chinese Yunnan and Guangxi armies crossed the border into Tonkin, occupying Lang Son, Bac Ninh, Hung Hoa and other towns. The French minister to China, Frédéric Bourée, was so alarmed by the prospect of war with China that in Novem