Final answer:
Congress responded to President McKinley's ultimatum by passing a joint resolution endorsing Cuban independence and allowing military intervention, which included the Teller Amendment to prevent U.S. annexation of Cuba, leading to the commencement of the Spanish-American War.
Step-by-step explanation:
After President McKinley's demands, Congress took swift action. On April 19, Congress passed a joint resolution recognizing Cuban independence and authorized the use of military force to remove Spain from the island. Included within this resolution was the Teller Amendment, ensuring that the United States would not annex Cuba post-war, a measure aimed at appeasing anti-expansionists.
This set the stage for the United States to enter the Spanish-American War, a conflict brief but decisive in its outcomes, confirming the U.S. as a burgeoning global power with newfound imperial possessions such as the Philippines.