Final answer:
The event that led to the naval blockade around Cuba in 1962 was the discovery of Soviet nuclear missiles in Cuba by U-2 spy planes.
Step-by-step explanation:
In 1962, the Cuban Missile Crisis escalated when aerial photographs taken by U-2 spy planes revealed the presence of Soviet nuclear missiles in Cuba. In response to the discovery of these missile sites, which posed a direct threat to United States territory, President John F. Kennedy ordered a naval blockade, referred to as a "quarantine" to avoid the implications of an act of war under international law. The blockade aimed to prevent Soviet ships from delivering additional missiles to the island. This tense period in the Cold War saw the world on the brink of nuclear conflict as the U.S. prepared for a possible invasion of Cuba if the Soviet Union did not remove the missiles.