Incendiary bombs were developed during World War I, coinciding with the advent of air vehicles in warfare. They were likely descended from a much longer tradition of fire weapons used in sieges and naval combat. German forces dropped thickened fuel bombs from zeppelins on cities beginning in 1915. Both sides would later develop bombs using metallic substances such as thermite and magnesium alloys that burn at very high temperatures and are difficult to extinguish. The light weight of incendiary devices also made them favorable compared to high explosive bombs.