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Military use of helmets declined after 1670, and rifled firearms ended their use by foot soldiers after 1700 but the Napoleonic era saw ornate cavalry helmets reintroduced for cuirassiers and dragoons in some armies which continued to be used by French forces during World War I as late as 1915.
World War I and its increased use of artillery renewed the need for steel helmets, with the French Adrian helmet and the British Brodie helmet being the first modern steel helmets used on the battlefield, soon followed by the adoption of similar steel helmets, such as the Stahlhelm by the other warring nations. Such helmets offered protection for the head from shrapnel and fragments.
From antiquity to the 17th century the helmets are very present although not everybody afforded them. At some point around 1670 they became useless against new firearms but kept their importance for cavalry, in part for decorative reasons, in part because cavalry remained more exposed to blade/sword attack. these are very conspicuous in Napoleonic era (early 19th century).
French Napoleonic Dragoon Helmet (1806 Pattern)
WW1 saw the rise of the "modern" steel helmet as a protection against omnipresent use of artillery with fragmentation shells.