 In 1789 , Nguyễn Ánh made Duyệt a general . From this point on , Duyệt accompanied his master on many military campaigns against the Tây Sơn . Continuous warfare then ensued , mostly centred near Nha Trang and Qui Nhơn on the south central coast , where the Nguyễn besieged the Tây Sơn 's strongholds . In 1801 , Duyệt engineered a naval victory in Thị Nại , which was a turning point of the war and heralded the complete collapse of the Tây Sơn . In the same year , a close colleague , Tong Viet Phuc was killed in battle by the Tây Sơn , leading Duyệt to fly into " an almost insane rage " and start killing every enemy soldier he came across , leading Nguyễn Ánh to rebuke him . Shortly thereafter , Nguyễn Ánh exploited the absence of most of the Tây Sơn 's army , which was attempting to recapture Quy Nhon , to lead his army in an attack on Phú Xuân , the capital of the Tây Sơn . However , the Nguyễn encountered heavy resistance from Tây Sơn forces around the Tu Dung sea gate , the entrance into the citadel of Phú Xuân . Seeing that the Nguyễn forces could not breach the defense by frontal attacks , Nguyễn Ánh ordered Duyet to lead a naval division to attack the Tây Sơn defense complex from the rear . Lê Văn Duyệt and his deputy Le Chat then defeated the Tây Sơn army and forced their commander , Prince Consort Nguyen Van Tri , to flee . This paved a way for the Nguyễn to assault the citadel of Phú Xuân . 
