President Chester Arthur appointed Root as U.S. attorney for the southern district of New York (1883-1885).  As chairman of the New York County Republican Party, Root worked on Theodore Roosevelt’s unsuccessful mayoral campaign in 1886 and his successful gubernatorial race in 1898.  The next year, President William McKinley surprised the press by selecting Root as his new secretary of war.  Although Root was not well known outside of New York and the legal profession, the president wanted a keen legal mind to deal with the questions arising from the United States’ acquisition of foreign territories following the Spanish-American War.  In Cuba, the new secretary of war encouraged economic development, sanitary and educational reforms, and road construction.  Uncertain about the ability of native Cubans to rule themselves, he insisted on adoption of the Platt Amendment, which promised eventual military withdrawal in return for recognition of the right of intervention by the American military in case law and order broke down.

In the Philippines, Root oversaw the U.S. military effort to suppress a native rebellion against American rule.  When anti-imperialists publicized atrocities committed by American soldiers in the Philippines, Root saw to it that the perpetrators were punished, while defending American military policy during the guerrilla war.  He also wrote instructions for the U.S. civilian commission that replaced American military rule on the islands, and which became the foundation of the 1902 law that set forth the process for the eventual return of political control to Filipino hands (commonwealth in 1934; independence in 1946).  Frustrations in dealing with the Filipino situation prodded him to undertake a substantial administrative reorganization of the War Department, despite the opposition of the army’s commanding general, Nelson Miles.  The reforms included more than doubling the size of the standing army, strengthening the department’s authority over the states’ National Guard, founding the Army War College in order to enhance the strategic ability of army officers, and centralizing responsibility for military policy through a general staff system.

In early 1904, Root resigned as secretary of war, but returned to the cabinet in late 1905 as President Roosevelt’s secretary of state.  Root worked diligently to improve U.S. relations with Latin America, touring the region in 1906 and supporting the Central American Peace Conference the next year.  He used his legal skills to negotiate 22 arbitration treaties, and in 1909 resolved a decades-old fisheries dispute between the United States and Canada.  Working effectively with the U.S. Senate, all the major treaties he submitted to that body were approved.  When a tense diplomatic dispute arose with Japan, the secretary of state worked out the “Gentlemen’s Agreement,” which became the basis for the Root-Takahira Agreement of 1908.  He also created the Division of Far Eastern Affairs within the State Department.  In 1912, Root was awarded the Nobel Peace Prize for his role in promoting peaceful arbitration.

In early 1909, Root resigned from the cabinet after the New York legislature elected him to the U.S. Senate.  He was dismayed when a personal and political breach developed between President William Howard Taft, with whom he had worked closely concerning the Philippines, and his old friend and former boss, Theodore Roosevelt.  However, Root sided politically with Taft and the more conservative Republicans.  When Root chaired the 1912 Republican National Convention that renominated Taft over the former president, Roosevelt never forgave Root.  The Republican split that year allowed Democrat Woodrow Wilson to win election.  Root voted against Wilson’s domestic legislative agenda, but backed the president’s efforts to repeal the toll exemption for American ships in the Panama Canal, which the senator considered a treaty violation.  Root opposed the 17^th Amendment (1913), which constitutionally mandated the direct election of U.S. senators, and retired when his term ended in March 1915.