William McKinley was the 25th president of the United States, serving from 1897 until his assassination in 1901. A member of the Republican Party, he led a realignment that made Republicans largely dominant in the industrial states and nationwide for decades. McKinley successfully led the U.S. in the Spanish–American War and oversaw a period of American expansionism, with the annexations of Hawaii, Puerto Rico, Guam, the Philippines, and American Samoa.