With Austria weakened, Bismarck now turned his attention to the other great stumbling block to unification - the French. France had watched Prussia's growing power with alarm. Bismarck tried to weaken France as much as possible before war started:
Bismarck found his excuse for war when Spain offered its vacant crown to Prince Leopold (a relative of Prussia’s Wilhelm I):
Napoleon III was advised that the French army could defeat Prussia. In France, his popularity was in decline. Military victory could restore opinion. French generals felt their army was better organised and better equipped than Prussia.
Prussia and her allies outnumbered the French. In reality the Prussian were better organised too. They were mobilised quickly and well supplied.
In contrast, the French were disorganised, with many late to reach the front. Their supplies were inadequate.
France was heavily defeated in the Franco-Prussian War. Napoleon III was overthrown by a French rebellion.
The circumstances leading to the war caused the southern German states to support Prussia. This alliance led to the unification of Germany.