Final answer:
President McKinley engaged Spain in order to secure peace and protect American interests, including the strategic prize of the Philippines.
Step-by-step explanation:
The key reason for engaging Spain, according to McKinley, was to secure peace while preserving American interests.
During the Spanish-American War, President McKinley was reluctant to relinquish the strategically useful prize of the Philippines and did not want another European power to seize them. By securing control over the Philippines, the United States aimed to protect American citizens and their property, as well as maintain a presence in the Pacific.
This engagement also had elements of humanitarian concerns mixed with self-interest, as McKinley believed that by adding the Philippines to the U.S. map, the American government could uplift and civilize the Filipinos.