How do the Russians defeat the Chinese and force them to be subjugated? Or the Chinese? Think about logistics. And, it is more likely the Tsar gets married to a Chinese princess after conquering Beijing while the alternative is impossible.
Marrying a Chinese princess isn't going to do anything.Women can't succeed the throne in China.Dynasties in China are passed entirely through the male line.If an outsider from the family takes over the throne,the country is finished,so no one will acknowledge the husband of the princess as emperor.
Well, how did the Qing defeat the Ming? How did the Mongols conquer the Russians? Its been done before.
Cavalry,good leadership,unity,politics and a lot of luck.The Ming Dynasty was essentially fighting a massive peasant rebellion(whose ranks reportedly had over a million soldiers).Instead of evacuating to the south when the rebels marched on Beijing,the emperor stayed and together with most of his family either committed suicide,captured or were executed when the officials and the city's garrisons defected over to the rebels.This caused a massive power vacuum where a lot of government officials and commanders were left with three options:defect to the rebels;join the Manchus;continue to serve whoever's acclaimed the new emperor in Nanjing or declare themselves a ruler.Critically,the commander in charge of the Shanhai pass(the gateway to China),Wu Sangui,chose to defect over to the Manchus with his troops after hearing his family was mistreated back in Beijing by the rebels.This allowed the Manchus to pass unmolested into China,defeated the rebels and received the defection of a large number of Ming soldiers(who believed that the Manchus are winning).After crushing the rebels,the Manchus were able to conquer what's left of Ming(commonly known as Southern Ming) due to power struggle between different contenders of the throne and various factions.