World War I
From Halopedia, the Halo wiki
World War I was the first human conflict in which flamethrowers were used. Trench warfare was common during the war.
By 1918, forces were told that the war would end on November 11. However, no orders were given about what would happen prior to that. This resulted in some forces standing down and others continuing to fight. On Armistice Day itself, the fighting was more intense than D-Day.
Following the conflict, flamethrowers changed very little over the centuries.
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