The longest battle of the war, the Battle of Verdun, is faught to a draw with an estimated one million casualties. (11h 40min, feb 21, 1916 y – 12h 20min, dec 18, 1916 y)
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Date:
11h 40min, feb 21, 1916 y
12h 20min, dec 18, 1916 y
~ 10 months