What role did William McKinley play in the Spanish–American War?

William McKinley was a former Congressman and Governor of Ohio before becoming the president of the United States in the election of 1896. McKinley had served in the Civil War and was generally thought highly by his superior officers for his actions as a soldier during the war. He had a national reputation as a capable legislator and for his work as governor.
McKinley took an interest in the revolution in Cuba almost immediately upon taking office. During this period of American history, the United States was in the national mindset of expanding American influence outside of the borders of the United States. Latin American countries and those located in the same hemisphere as the United States were looked upon as potential areas where the United States could flex diplomatic and military muscle, making the United States a player in international affairs. Cuba is approximately one hundred miles from the tip of Florida and drew particular interest from McKinley.
In February of 1898, the USS Maine (an American battleship) was thought to have hit a mine while entering into the port at Havana Cuba. The vessel sunk, and as you might imagine, the calls for retaliation rang loudly throughout the media in the United States. Spain was identified as the culprit and with requests for Congress to go to war McKinley was faced with a critical decision; go to war or negotiate a peaceful settlement by waiting to see more evidence of what caused the catastrophic explosion killing two hundred and sixty American sailors.
McKinley attempted to reconcile and negotiate with Spain to avoid war. No doubt, his experience in the Civil War influenced his thinking that the ends of war may be justified, but the means have catastrophic negative consequences on civilians and military alike. Spain had a reputation for having if not the finest fleet of naval vessels in the world certainly one of the most formidable. Uncertainty about the untested naval power of the United States may have also influenced McKinley to consider the possibility of avoiding war through diplomatic means. The evidence of the explosion on the USS Maine was determined by Naval Intelligence the result of an explosive mine, but no determination was made as to who had placed the mine in the harbor.
In the end, the public sentiment to retaliate for the sinking of the USS Maine resulted in a declaration of war. The sinking of the USS Maine was only part of the justification for war, as the Spanish had zealously suppressed the Cuban rebellion with aggressive military tactics that claimed the lives of innocent civilians as well as members of the rebel forces. American corporations had numerous investments in Cuba at the time, and it was thought the only way to protect those investments was to use military force. The Spanish–American War began and ended in about one hundred days, with the Spanish naval and military forces soundly defeated. The United States gained control over Cuba, Guam, the Philippines, and Puerto Rico.
Although McKinley tried to peacefully negotiate a pre-war settlement, he ended up successfully presiding over a war. As a footnote, McKinley's reluctance to immediately declare war for the sinking of the USS Maine was justified by historians and a group of naval investigators. In 1976, using modern forensic techniques, naval investigators determined the USS Maine sunk as the result of a fire and explosion in the ammunition lockers of the ship, not from contact with an explosive mine.
http://www.loc.gov/rr/hispanic/1898/mckinley.html

https://www.history.com/this-day-in-history/the-maine-explodes

https://www.newspapers.com/topics/american-imperialism/sinking-uss-maine/