Read this article about the technology of warfare during World War I. Although some of this technology had already been invented, it was the first truly mechanized war.
Flamethrowers
Defensive use
The Imperial German Army
deployed flamethrowers (Flammenwerfer) on the Western Front attempting
to flush out French or British soldiers from their trenches. Introduced
in 1915, it was used with greatest effect during the Hooge battle of the
Western Front on 30 July 1915. The German Army had two main types of
flame throwers during the Great War: a small single person version
called the Kleinflammenwerfer and a larger crew served configuration
called the Grossflammenwerfer. In the latter, one soldier carried the
fuel tank while another aimed the nozzle. Both the large and smaller
versions of the flame-thrower were of limited use because their short
range left the operator(s) exposed to small arms fire.