As the crisis intensified, many Americans feared that war, possibly nuclear war, was probable. The U.S. naval quarantine went into effect October 24. Initially, Khrushchev ordered Soviet ships to race toward the quarantine line. The Soviets threatened to sink any U.S. vessel that tried to prevent their passage to Cuba. That same day, the government put U.S. nuclear forces on DEFCON 2 alert for the first and only time in history: bombers remained airborne, and missile silo covers were opened.