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^BFirst World War munitions factory workers.^b Female workers training in the use of pneumatic hammers in a US munitions factory during the First World War (1914-1918). The First World War caused immense social upheaval as women entered the workforce in large numbers to replace the men drafted or conscripted to fight as soldiers. The USA entered the war on 6 April 1917, and over 4 million men were drafted to fight. The female workers played a vital role in producing munitions and keeping industries going. After the war, this social change aided the campaign for increased rights for women. This is the Midvale Steel and Ordnance Company, in Nicetown, Pennsylvania, USA. Photographed in 1918. (KEYSTONE/SCIENCE PHOTO LIBRARY/US ARMY/SCIENCE PHOTO LIBRARY)