They executed the outspoken rebel, and then went ahead and raped his widow Trac because why not.
OK, see this? This is my ass kicking face.
We now take it as pretty common knowledge that you just don't mess with Vietnam, even if you're an immensely powerful and technologically superior nation. They know kung fu, or something, and apparently every time they get invaded they simply level up.
Due to their traditional Confucian teachings, the Han Chinese considered women to be pretty useless. So they didn't realize that Trac and Nhi had been schooled in badass martial arts since they were little girls and thus weren't prepared to take any of Confucius' shit.