Yin and yang can be thought of as complementary (rather than opposing) forces that interact to form a dynamic system in which the whole is greater than the assembled parts. Everything has both yin and yang aspects, for instance shadow cannot exist without light. Either of the two major aspects may manifest more strongly in a particular object, depending on the criterion of the observation.
The Boxer Rebellion was a violent anti-foreign and anti-Christian movement which took place in China towards the end of the Qing Dynasty between 1898 and 1900. It was initiated by the Yihetuan, known in English as the "Boxers", and was motivated by proto-nationalist entiments and opposition to foreign imperialism and Christianity.