Before the war, Bismarck diplomatically isolated France in Europe so that she had no ally to assist her. For example, he isolated France from the three Southern German states of Bavaria, Baden and Wurttemberg by making them sign a military alliance with Prussia. He revealed to them that Napoleon III had a plan to annex them to France.
He also isolated France from Austria. This was done by the lenient terms of the treaty of Prague 1866. Prussia did not want permanent enemity with Austria and therefore did not annex any territory from Austria. Austria was just made to surrender the German states to Prussia. Austria therefore could not support France.
France was also isolated from Italy. Already Bismarck had assisted Italy to get Venetia in 1866. To make it worse, France still occupied Rome which the Italians badly needed to complete their unification.
Bismarck isolated France from Britain. He did this by publishing the 1865 proposal by France to annex Belgium which was under the guardianship of the British. This annoyed Britain and therefore there was way no she could support France against Prussia. Russia had already been isolated by Bismarck from any country against Prussia. Therefore, France had no friend to assist her which contributed to the victory of Prussia over France.