Final answer:
The Bay of Pigs invasion was deemed a colossal mistake by President Kennedy due to its utter failure, which led to international embarrassment for the U.S., consolidated Castro's power, and set the stage for the Cuban Missile Crisis.
Step-by-step explanation:
President Kennedy referred to the Bay of Pigs invasion as "a colossal mistake" and "the worst experience of [his] life" because it was a major failure for U.S. foreign policy. The intended coup was meant to replace Castro's government in Cuba with one more amenable to US interests. However, when approximately 1,400 Cuban exiles armed and backed by the U.S. landed at the Bay of Pigs on April 17, 1961, they were quickly defeated by the Cuban army due to a multitude of miscalculations and failures.
The Kennedy administration, having inherited the plan from the Eisenhower administration, erroneously believed that the Cuban people would support the invasion and rise up against Castro. This misconception, combined with Kennedy's last-minute decision to withhold air support from the invading exiles to avoid domestic and international repercussions, led to a swift defeat. The catastrophe not only embarrassed the U.S. globally but also strengthened Cuban resolve and led to closer ties between Cuba and the Soviet Union, paving the way for the Cuban Missile Crisis, a subsequent and even more dangerous standoff.