 In 1802 , Nguyễn Ánh , who had declared himself Emperor Gia Long after capturing Phú Xuân ( Huế ) , appointed Duyệt to the position of Khâm Sai <unk> Tả Quân Dinh Bình Tây <unk> Quân ( " Marshal of King 's Left Division , Tây Sơn Pacification General " ) and ordered him to attack Tây Sơn @-@ controlled northern Vietnam . In October 1802 , Duyệt captured the north , and then renamed it from Bắc Hà ( " Northern River " ) to Bắc Thành ( " Northern Citadel " ) thus marking ultimate victory of the Nguyễn against the Tây Sơn . 
