Everyone was building up their military so that they would be bigger than the competition. Eventually, things were so tense due to everyone building bigger and bigger weapons and enlisting more soldiers. Things just blew up from there.
The Franco-Prussian War (July 1870 - May 1871) was a major reason for the rise in militarism in Europe in the decades leading up to World War 1. This war ended as a crushing defeat of the French by several unified German states.
