China, a History

Yangdi gained the throne after his father's death (possibly by murder). He further extended the empire, but, unlike his father, he did not seek to gain support from the nomads. Instead, he restored Confucian education and the Confucian examination system for bureaucrats. By supporting educational reforms, he lost the support of nomads. He also started many expensive construction projects such as the Grand Canal of China. This combined with his failed invasions into Korea (with Chinese casualties exceeding well over 2 million in all the wars combined), invasions into China from Turkic nomads, and his growing life of decadent luxury at the expense of the peasantry, he lost public support and was assassinated by his own ministers.

Both Wendi and Yangdi sent military ventures into Vietnam as well, as northern Vietnam had been incorporated into the Chinese empire during the previous Han Dynasty (202 BC - 220 AD). However, the ancient Kingdom of Champa in southern Vietnam became a major contestant to Chinese invasions to its north. These invasions became known as the Linyi-Champa Campaign (602-605 AD). According to Ebrey, Walthall, and Palais:

The Hanoi area [that the Han and Jin dynasties had held] was easily recovered from the local ruler in 602, and a few years later the Sui army pushed farther south. When the army was attacked by troops on war elephants from Champa (in southern Vietnam), Sui feigned retreat and dug pits to trap the elephants. The Sui army lured the Champan troops to attack, then used crossbows against the elephants, causing them to turn around and trample their own army. Although Sui troops were victorious, many succumbed to disease, as northern soldiers did not have immunity to tropical diseases such as malaria.