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.illiam 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.