Many Chinese resented the extensive foreign intervention in China. This discontent coalesced into the nucleus of a secret group called the Righteous Harmony Society (義和團 – Yìhétuán); Westerners called them Boxers. The Boxers, with the support of Prince Duan, undertook a campaign against foreign influences and foreigners. By early 1900 this movement had brought much of China to the verge of revolution. Boxers in the northern provinces attacked and killed hundreds of Chinese Christians and foreigners, mostly missionaries. The wave of unrest came to an apex following the assassination of Clemens von Ketteler, the German ambassador plenipotentiary, on June 20. The Chinese government then broke off diplomatic relations and demanded that all foreigners leave immediately, promising safe escort. About 3,500 foreigners and Chinese Christians refused to leave, and fearing for their safety, took refuge in the foreign legation compound, where they were besieged. A composite military force of 407 men (including 56 Americans) plus about 200 civilians defended the compound.