Who started the Boxer Rebellion?

The Boxer Rebellion was started by a clandestine nationalist organization called the society of the Righteous and Harmonious Fists in 1900. They initially led an uprising in Northern China against Western and Japanese influence. The Boxers (so-called because of the physical exercises they performed) went on the rampage, killing a number of Westerners and Japanese, as well as destroying property. The Boxers, like many Chinese, blamed foreigners for their poverty and lack of opportunity. Foreigners had been given lucrative trade concessions which effectively cut Chinese traders out of the market. The Boxers and their supporters felt that their country was being colonized by stealth.
During the long, hot summer of 1900, the Boxer Rebellion erupted in full, spreading to the capital, Beijing. From June to August 1900, the Boxers besieged that part of the city housing the various diplomatic missions and embassies. The following day, the Empress Cixi, a Boxer sympathizer, declared war on all foreign countries with diplomatic links to China. In response, Japan and the Western powers assembled a formidable armed force to crush the rebellion, which it did. An armed insurrection had been successfully put down, but a growing Chinese nationalism, fanned by the flames of resentment at colonial exploitation, merely simmered underground, ready to burst forth at the next opportunity.
https://www.britannica.com/event/Boxer-Rebellion