In 1866, under the breath of ministerial arguments and oratorical onslaughts Napoleon III.'s policy weakly wavered, thereby giving to Bismarck's statecraft a decisive triumph all along the line.
In vain did he in the latter part of that year remind the Prussian statesman of his earlier promises (always discreetly vague) of compensation for France, and throw out diplomatic feelers for Belgium, or at any rate Luxemburg[8].

In vain did M.Thiers declare in the Chamber of Deputies that France, while recognising accomplished facts in Germany, ought "firmly to declare that we will not allow them to go further" (March 14, 1867).

Bismarck replied to this challenge of the French orator by publishing five days later the hitherto secret military alliances concluded with the South German States in August 1866.
Thenceforth France knew that a war with Prussia would be war with a united Germany.
[8] In 1867 Bismarck's promises went so far as the framing of a secret compact with France, one article of which stated that Prussia would not object to the annexation of Belgium by France.

The agreement was first published by the _Times_ on July 25, 1870, Bismarck then divulging the secret so as to inflame public opinion against France.
In the following year the Zollverein, or German Customs' Union (which had been gradually growing since 1833), took a definitely national form in a Customs' Parliament which assembled in April 1868, thus unifying Germany for purposes of trade as well as those of war.

This sharp rebuff came at a time when Napoleon's throne was tottering from the utter collapse of his Mexican expedition; when, too, he more than ever needed popular support in France for the beginnings of a more constitutional rule.