Ka kammala wannan magana: A lokacin da Yaƙin Duniya na Ɗaya ya ɓarke a shekara ta 1914, Jacob ya tafi tare da rundunar Meerut, wani ɓangare na rundunar sojojin Indiya, zuwa Yammacin Yamma. Ya ga aikin a matakan karshe na
Yaƙin La Bassée a watan Oktoba 1914. An ba shi mukamin brigadier-general na wucin gadi a ranar 5 ga Janairu, 1915, an nada shi ya zama kwamandan Dehra Dun Brigade, kuma ya jagoranci brigade a yakin Neuve Chapelle a watan Maris na 1915 da yakin Aubers Ridge a watan Mayu na 1915. An daukaka shi zuwa babban janar na wucin gadi a ranar 7 ga Satumba, 1915, ya zama Janar Jami'in Kwamandan rundunar Meerut kuma ya jagoranci rundunar a yakin Loos a watan Oktoba 1915. Tare da Corps na Indiya suna shirin barin Yammacin Yamma, an nada shi don karɓar rukunin 21 na "Sabbin Sojoji" a ranar 18 ga Nuwamba, 1915. An gabatar da shi zuwa babban mukamin babban janar a ranar 1 ga Janairu 1916 kuma duk da cewa ya ji rauni a watan Maris na 1916, an ba shi mukamin laftanan janar na wucin gadi a ranar 28 ga Mayu 1916 kuma an nada shi ya zama kwamandan rundunar II a cikin Sojoji na Biyar a watan Satumbar 1916. A lokacin yakin Somme, rundunarsa ta dauki harin Birtaniya a lokacin yakin Thiepval Ridge a watan Satumbar 1916 da kuma hare-haren da aka kai a St Pierre Divion a lokacin yakin Ancre a watan Nuwamba 1916. Ya kasance a cikin umurnin II Corps, bayan da aka ci gaba da zama babban jami'in janar a ranar 3 ga Yuni, 1917, don yakin Passchendaele a cikin 1917.