In World War II, many nations installed air raid sirens in cities to warn when enemy planes were coming to attack with bombs. After the war ended these sirens were kept ready to sound, in case a new war started, due to the tense relationship between the United States and Soviet Union (USSR) (Now Russia) that existed from 1947–1991. As weapons have gotten faster, quieter and harder to spot, the ability to provide warning before a bomb or missile falls has become difficult. A number of countries, including the United States, now use their sirens to warn of severe weather, such as tornadoes, tsunamis; emergencies at dams, chemical plants and nuclear power stations.