But each triumph weakened their army and its morale .
In their most successful attack yet they advanced 12 km ( 7.5 mi ) on the first day , crossing the Marne but stopping 56 kilometres ( 35 mi ) from Paris .
Each triumph weakened their army and its morale .
In their most successful attack yet they advanced 12 km ( 7.5 mi ) on the first day , crossing the Marne but stopping 56 kilometres ( 35 mi ) from Paris .