With the adoption of sky blue uniforms and steel Adrian helmets in 1915 to replace the conspicuous peace time uniforms worn during the early months of war, the kepi was generally replaced by folding forage caps. Officers however still wore their kepis behind the lines., French police (since replaced with caps), French Foreign Legion, Unrelated RATP hat, French Gendarmerie, French Army. Following the war the kepi was gradually reintroduced in the peacetime French army. The Foreign Legion resumed wearing it during the 1920s; initially in red and blue and then in 1939 with white covers on all occasions. The bulk of the French army readopted the kepi in the various traditional branch colours for off-duty wear during the 1930s. It had now become a straight sided and higher headdress than the traditional soft cap. This made it unsuitable for war time wear and after 1940 it was seldom seen being worn except by officers. An exception was the Foreign Legion who, previously just one of many units that wore the kepi, now adopted it as a symbol.
Source: Wikipedia > Kepi
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