How was France a threat to Germany in WWI?

France had ceded the provinces of Alsace-Lorraine to the newly unified Germany after the disastrous Franco-Prussian War of 1870-71. France wanted revenge for their defeat, and to regain the 2 lost provinces.However, France was not strong enough to fight Germany by itself, and knew this.They therefore had to look for Allies, and the desire for revenge was so strong that eventually they were bound to build up alliances which would mean Germany becoming more and more isolated, and threatened with war on 2 fronts and against an alliance of major powers.