William McKinley. William McKinley was the 25th President of the United States, serving from March 4, 1897, until his assassination on September 14, 1901, after leading the nation to victory in the Spanish-American War and raising protective tariffs to promote American industry.ublic indignation brought pressure upon the President for war. Unable to restrain Congress or the American people, McKinley delivered his message of neutral intervention in April 1898. Congress thereupon voted three resolutions tantamount to a declaration of war for the liberation and independence of Cuba.