He was one of the most prominent administrators in the history of America. He believed that international law and its mechanisms offered the greatest chance for securing peace. However, as a tough realist, he knew that it would take time, wisdom and patience to achieve peace and that its effective implementation would require many efforts.

He was a Republican. Between 1889 and 1904, he served as Secretary of War. He reorganised the War Department, enlarged the military academy at West Point and established the General Staff. He designed the American policies for Cuba and the Philippines and continued the Open Door Policy in the Far East. In 1905, President Theodore Roosevelt appointed him Secretary of State.

During World War I, he opposed the policy of neutrality adopted by the US. He served on a League of Nations commission which created the Permanent Court of International Justice. Between 1910 and 1925, he was president of the Carnegie Endowment for International Peace and helped establish its European counterpart. As an ambassador extraordinary, he also headed a mission to Russia in 1917.