Resentment of foreign intervention crystallized in the establishment of I-ho ch'üan (Righteous and Harmonious Fists), called Boxers in the West because of their belief that mystical boxing rituals protected them from bullets.  The Boxers were primarily a religious society that initially focused its wrath on Christian missionaries and Chinese converts to the Western religion.  Their agenda soon expanded into the eradication of all foreign presence and influence in China, and they attracted strong backing in Northern China, which had been devastated by floods and drought.  

In 1898, the Boxers led a rebellion in Shantung province and soon gained adherents in the Chinese capital of Peking (Beijing).  The ruling Manchu court was ambivalent about the movement, pleased by its anti-foreign drive but concerned about its destabilizing affect on China, and took a neutral stance at first.  However, by the spring of 1900, the Ch'ing administration gave its secret blessing to the Boxers.  In early June, an international force of 2000 sailed from Tientsin to Peking, where the Boxers were burning foreign property and killing foreign nationals and Chinese Christians.  Meanwhile, the Empress Tz'u-hsi declared war on the foreign powers.

As associates ran McKinley's reelection campaign, the president and his foreign policy advisors crafted America's response to the Boxer Rebellion.  The administration preferred the United States to act independently, but circumstances soon prodded McKinley to order the American military commander in China, Rear Admiral Louis Kempff, to "act in concurrence with other powers so as to protect all American interests."  In late June 1900, McKinley transferred 2500 American soldiers from the Philippines, where they were suppressing an uprising against American control, to China.  The troop dispatch sparked criticism from American politicians (mainly Democrats) and editors who charged the president with imperialism and exceeding his Constitutional authority.  McKinley believed a president's Constitutional war powers granted him such authority.

American soldiers were among the 20,000 foreign troops who ended the Boxer's siege of Peking on August 14, 1900.  Secretary Hay had convinced McKinley, who earlier expressed interest in the U.S. gaining a "slice" of China, to reiterate the Open Door policy with a diplomatic memorandum to "preserve Chinese territorial and administrative entity, protect all rights guaranteed to friendly powers by treaty and international law, and safeguard for the world the principle of equal and impartial trade with all parts of the Chinese Empire."  Its idealistic rhetoric made overt disagreement by domestic and foreign interests difficult, but its implementation was far more problematic.  The Boxer Rebellion undermined the prestige of the Ch’ing administration, and China initiated another series of reforms in 1901, but the foreign presence and influence continued for decades.