Vietnamese over history have displayed immense ingenuity in spite of not sharing a language with its northern neighbor whose imperial examination system produced a scholar-bureaucrat class of scientific researchers and publications. Historically Vietnamese innovation has been best demonstrated during wartime, in which Vietnamese developed on top of native, Chinese, Western and other inventions and military tactics. Brave Vietnamese military leaders and warriors developed practical wartime strategies and technology to deal with Chinese intruders, for example at Battle of Bạch Đằng in 981 when Ngô Quyền developed an original technique of sinking Chinese navy fleets. The same original technique of sinking naval fleets developed by Ngô Quyền was later employed by Vietnamese generals to repulse failed invasions by the Mongol Empire, an empire that would later go on to ravage much of Russia, Europe, China and the Arab world. In the 15th century Vietnamese scholar Hồ Nguyên Trừng was known for his development innovations on firearms and cannons and helped transfer knowledge of Vietnamese firearm technology once living in exile in China. Firearm and cannon technology developed by the Vietnamese would prove vital to the conquest of Champa by the Đại Việt in the Cham-Vietnamese War of 1471 and overall Vietnamese matchlocks during this period were considered world-class not only with Ming China but also European observers of the Trịnh–Nguyễn civil war. By the 1800s, Nguyễn rulers such as Emperor Minh Mạng claimed the legacy of Confucianism and China's Han dynasty for Vietnam in relation to its neighbouring states.