Kammala jumla mai zuwa: Pétain ya jagoranci rundunarsa a yakin Guise (29 ga Agusta 1914). A ƙarshen watan Agusta 1914 an daukaka shi da sauri zuwa brigadier-general kuma an ba shi umarnin Rukunin na 6 a kan lokaci don Yaƙin farko na Marne; kadan fiye da wata daya daga baya, a watan Oktoba 1914, an sake inganta shi kuma ya zama kwamandan rundunar XXXIII. Bayan ya jagoranci rundunarsa a lokacin bazara na 1915 Artois Offensive, a watan Yulin 1915 an ba shi umarnin Sojoji na Biyu, wanda ya jagoranci a cikin
Yaƙin Champagne a wannan kaka. Ya sami suna a matsayin daya daga cikin kwamandojin da suka fi nasara a Yammacin Yamma.