A: The United States' monopoly on nuclear weapons was broken in 1949 when the Soviet Union tested its first nuclear device, and many in the government and public perceived that the United States was more vulnerable than it had ever been before. Duck-and-cover exercises quickly became a part of Civil Defense drills that every American citizen, from children to the elderly, practiced to be ready in the event of nuclear war. In 1950, during the first big Civil Defense push of the Cold War; the movie Duck and Cover was produced (by the Federal Civil Defense Administration) for school showings in 1951. At the time, it was believed the main dangers of a Hiroshima-type nuclear blast were from heat and blast damage: to maximize material damage, the air-burst was the preferred nuclear tactic which maximized surface damage and resulted in fallout being dispersed into the stratosphere. In the Operation Crossroads tests of 1946, Test Able (air burst) had little local fallout, but Test Baker (underwater) left the test targets badly contaminated with radioactive fallout, leading to cancellation of Test Charlie. Widespread radioactive fallout itself was not clearly recognized as a threat until 1954, after the Castle Bravo nuclear-weapon test in the Marshall Islands caused sickness and death in Japanese fishermen on the Lucky Dragon fishing vessel far outside the planned test area.