Rubuta ci gaba don wannan sakin layi - A cikin Janairu 1915, kwana ɗaya kafin a kira shi zuwa aikin soja a Yaƙin Duniya na I, ya auri Rosa Koch. An tura shi zuwa ga bindigogi a yammacin gaba, inda ya zauna har zuwa karshen yakin, ya shiga cikin yakin Champagne
(1915<unk>1916), Yaƙin Somme (1916), Yaƙin Arras (1917), Yaƙin na biyu na Aisne (1917), Yaƙin Cambrai (1917) (1917), da Yaƙin Soissons (1918). Don hidimarsa, Thälmann ya karɓi Gicciye na Iron Class na Biyu, Gicciye na Hanseatic da Alamar Rauni (sau biyu).