Airplanes probably ended trench warfare because they could fly over enemy camps, fire down on them, and gather information about troop arrangements.
Source: My social studies teacher (a little) and my inference...
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The machine gun changed warfare
mustard gasthe tanktrench warfare. (although begun in the American Civil War)
well it can kill just about anything that is alive you know. that's why people hat it when one falls on them ^_^
If you mean used it in an act of warfare, as opposed to a test, the only country to do so has been the USA (Hiroshima and Nagasaki, 1945)
The use of chemical weapons during the first World War ; see relevant link below .