This study examines the military role of the 3,000 Long Mon soldiers in the history of Vietnam. These Ming refugees have been understood as the remnants of Ming troops against Manchurian invasion of China, though their career in China as pirates have also been known in previous studies. Official view on these Ming refugees of the Nguyen dynasty is that they were sent to My Tho and Bien Hoa, and that they opened arable land, built market places to attract overseas traders, and the spread of Han or Hoa air began by their settlements. In other words, the economic development of My Tho and Bien Hoa area was attributed to the contribution of the 3,000 Ming refugees. However, author of this study claims that the main contribution of these Ming refugees was not economic development of ``Lower Cambodia`` region, but the territorial expansion of Vietnam by the military means. As the background of the conflicts between Vietnam and Cambodia, chapter one of this article reviews the military intervention of Vietnam to the Cambodian civil wars based on the Vietnamese official records. By examining the operation routes of Vietnamese troops, author made a clear conclusion that the region from Bien Hoa to the west had not been the part of Vietnamese territory until the Vietnamese court sent the 3,000 Ming soldiers to My Tho and Bien Hoa in 1679. In the chapter two, author examines the Ming refugee group in My Tho. Among the leading figures of this group, power struggle occurred. In 1688, the vice commander Hoang Tan assassinated the commander Duong Ngan Dich. Hoang moved his soldiers to the upper area of the Mekong causing Cambodian king to doubt Vietnamese invasion of Cambodia started. The commander of the Vietnamese troops ended Hoang``s leadership and put the soldiers of Hoang under the leadership of Tran Thuong Xuyen, another leader of Ming refugee solders who had stationed in Bien Hoa. The chapter three traces Tran Thuong Xuyen and his soldiers who were called Long Mon soldiers - tuong si Long Mon by the court chronicle of Vietnam. From 1689 to 1730s, the Long Mon soldiers were found in every war between Vietnam and Cambodia as the form of the vanguard under the command of Tran Thuong Xuyen. The contribution of Tran Thuong Xuyen and his soldiers was impressive until the Bien Hoa and Ha Tien were connected together by the new lands of Vietnam in ``Lower Cambodia`` in 1732. Author concludes that the role of the 3,000 Ming refugees in the field of agriculture and trade was not as big as the court chronicle claims. More contribution of the soldiers was in the field of military operation to convert the most of the lands of Lower Cambodia to those of Vietnam. They were properly used as the tool of oppressing the resistance of Cambodia, a barbarian - man- country from the point of Vietnamese view. But to the eyes of Vietnamese court, the Long Mon soldiers also could have been another barbarians if we remember Vietnamese tactic of ``di man cong man (to strike barbarians by barbarians)`` The fact that the Ming refugees had been pirates in the South China Sea and the Gulf of Tonking was an enough reason for the Vietnamese kings to regard the Chinese soldiers as to be man.