Germany formed alliances because Prussian Chancellor Bismarck felt that German/Prussian honour was being offended.
So Prussian army went to war with France on 19 July 1870 and won the war on 10 May 1871.This war became famous as Franco-Prussian war in history won by Prussia and led to unification of Germany.
The Franco-Prussian War (1870) was a conflict between the Second French Empire and Third French Republic, and the states of North German Confederation led by the Kingdom of Prussia. It resulted in a German victory and it was a key part of the wars of Unification of Germany made specifically to building an integrated nation state from the former Empire. This ultimately set the precedents for Nazi Germany and World War II.
