Who was John Hay and what was his public service career like? How and why was he an influential American?

John Hay enjoyed a long career in public service. He was one of Lincoln's personal secretaries. He also helped write a biography of Lincoln with his fellow secretary John Nicolay which shaped the president's image among historians of that era. John Hay served as Ambassador to Great Britain in 1897 and was instrumental in smoothing over ill will between the United States and Britain over British involvement during the Civil War on behalf of the Confederacy. Hay also received British assurances that Britain would not intervene during the Spanish-American War. Hay served as Secretary of State from 1898 to 1905. His greatest achievement in office was the creation of the Open Door Policy in China which ensured China's territorial sovereignty at a time when Europe and Japan sought out their own sphere's of influence in the country. Hay also played a prominent role in the Boxer Rebellion, asking only that guilty Chinese be punished for the attacks against Westerners. Hay was also instrumental in predicting Panama's war of independence against Colombia and in attaining American rights to build a canal in Panama.
John Hay's career is noted for longevity as well as making the United States a dominant player in the Western Hemisphere. Hay was fortunate to be a major player in public life under the McKinley and Roosevelt administrations when the United States wanted to expand its international outreach.
https://history.state.gov/departmenthistory/people/hay-john-milton