Final answer:
The Spanish-American War in 1898 marked the beginning of U.S. imperial expansion with the annexation of Puerto Rico, Guam, and the Philippines, and compromised independence of Cuba through the Platt Amendment.
Step-by-step explanation:
The Spanish-American War was a pivotal moment in United States history, marking a turn toward imperialism. Sparked by the explosion of the USS Maine, the event witnessed a surge in yellow journalism that influenced public sentiment towards war. The conflict led to the annexation of Puerto Rico, Guam, and the Philippines, while Cuba gained a form of independence that still carried heavy US influence through mechanisms like the Platt Amendment.
Initially, President McKinley was reluctant to engage in the Cuban War of Independence, even as the situation destabilized. However, the mysterious explosion of the USS Maine in Havana Harbor on February 15, 1898, falsely attributed to Spanish aggression, swung U.S. opinion in favor of intervention. As a result of the war, Spain relinquished control over Cuba and ceded Puerto Rico, Guam, and the Philippines to the United States. Following the treaty, Cuba's 'independence' was compromised by the Platt Amendment, which allowed for substantial U.S. influence in the new republic.
U.S. interest in the conflict was fueled by economic investments and the strategic desire to expand American influence overseas. The war's outcome significantly reduced the Spanish Empire's presence in the Western Hemisphere and marked the beginnings of a U.S. global presence through territorial acquisitions, thus signalling the emergence of the U.S. as an imperial power.