An active Civil Defense plan in times of war reduces loss of life and property resulting from enemy actions. During the Second World War the threat of aerial attack on cities was sufficient enough that many cities implemented organized civil defense strategies. Citizens were trained in fire fighting, rescue, medical first-aid and shelters were built to protect against bombing raids and other attacks. Other tactics included power blackouts that significantly reduced the glow of city lights, typically used as guides by enemy pilots in bombing campaigns. The introduction of nuclear weapons, capable of destroying entire metropolitan areas, caused radical changes in how civil defense is now approached, including: plans for mass evacuation, and prolonged residence in shelters.