Rubuta ci gaba don wannan sakin layi - Rukunin Tsaro na 21 ya yi yaƙi a Yammacin Yamma, yana shiga cikin ƙaddamar da hare-haren Jamus wanda ya haifar da Babban Maƙasudin Allied kuma ya ƙare tare da Yaƙin Marne na farko. Bayan haka, sashen ya kasance a cikin layi a yankin Champagne har zuwa Yuni 1916. A watan Yulin 1916, rukunin ya shiga yakin Verdun. Ya koma Champagne a watan Satumba sannan ya koma Verdun a watan Disamba 1916-Janairu 1917. A cikin bazara na 1917, rukunin ya yi yaƙi a karo na biyu na Aisne
, wanda kuma aka sani da Yaƙin Champagne na Uku. A watan Nuwambar 1917, rukunin ya ga aikin a yakin tanki a Cambrai. Rukunin ya shiga cikin hare-haren bazara na Jamus na 1918. Ya fuskanci Allied Hundred Days Offensive, kuma ya yi yaƙi a Yaƙin Somme na Biyu (1918) (wanda kuma ake kira Yaƙin Somme na Uku). Ƙungiyar Allied Intelligence ta ƙayyade sashen a matsayin na biyu.