The tank is a British invention and was used by the allied troops to break that dreadful stalemate in World War I. Tanks easily rolled over the fortified trenches and pushed the German troops back.
The Allies were actually not too impressed by this new weapon, since those tanks were a nuisance: they had many technical problems and were expensive. It were the German generals that were impressed and pushed tank warfare to new heights: the Blitzkrieg.
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