Answer:
Germany feared that France wanted to recapture Alsace and Lorraine.
Step-by-step explanation:
The Franco-Prussian War was fought between the Second French Empire and the Kingdom of Prussia over the extension of the German territory. The war came to an end with the sign of the Treaty of Frankfurt in 1871 which gave Germany the French territories of Alsace and Lorraine.
France wanted its territories back so, because of the fear of another Franco-Prussian War is that Germany started to build up its military.
It was a key conflict for what it would come later, World War I.
I hope this answer helps you.