Trench warfare
Trench warfare has become archetypically associated with the First World War of 1914–1918, when the Race to the Sea rapidly expanded trench use on the Western Front starting in September 1914. By the end of October 1914 the whole front in Belgium and France had solidified into lines of trenches, which lasted until the last weeks of the war. Mass infantry assaults were futile in the face of artillery fire as well as rapid rifle and machine-gun fire. Both sides concentrated on breaking up enemy attacks and on protecting their own troops by digging deep into the ground. Trench warfare also took place on other fronts, including in Italy and at Gallipoli.
Trench code
The Germans started using trench codes in the spring of 1917, evolving into a book of 4,000 codewords that were changed twice a month, with different codebooks used on different sectors of the front. The French codebreakers were extremely competent at cracking ciphers but were somewhat inexperienced at cracking codes, which require a slightly different mindset. It took them time to get to the point where they were able to crack the German codes in a timely fashion.