By the end of the 1800's, the Western powers and Japan had forced China's ruling Qing Dynasty to accept wide foreign control over the country's economic affairs. They fought several wars: the Opium Wars (1839-42, 1856-60) and the Sino-Japanese War (1894-95), to try to resist the foreign influence in their country. China lacked modernized military and suffered millions of casualties.
In the late 1890's, a Chinese secret organization called The Righteous and Harmonious Fists, who performed regular attacks on foreigners and christians. They were very well-trained and they practiced martial arts and calisthenics (how the nickname "Boxers" was created).
In 1900, the Boxer movement spread to Beijing, where they destroyed churches and railroad stations. On June 20th, 1900, they began a siege on Beijing's foreign district, where the official quarters of diplomats are located. The next day, Qing Empress Dowager Tzu'u Hzi (or Cixi) declared a war on all foreign nations with diplomatic ties in China.
As Japan and the Western Powers organized a multinational force to crush the rebellion, the siege stretched into weeks, and the diplomats suffered through hunger and degrading conditions as they fought to keep the Boxers at bay. Several hundred foreigners and several hundred thousand Chinese Christians died.
On August 14th, an international force of approximately 20,000 troops from many nations arrived in Beijing to rescue the foreigners and Chinese Christians.
The Boxer Rebellion formally ended with the signing of the Boxer Protocol on December 7th, 1901. It said China had to take down forts protecting Beijing, Boxers and Chinese government officials involved in the uprising were punished, China couldn't buy arms for two years, and they had to pay over 300 million dollars in reparations to foreign nations.
The Qing Dynasty was weakened by the Boxer Rebellion, and ten years, the dynasty came to an end.