Then, too, the Crimea, Magenta, and Solferino looked less brilliant since this transforming seven-weeks' war, behind which stood Bismarck with his wide-reaching plans.
His own magnificent scheme of a Hapsburg empire in Mexico under a French protectorate had failed, and now there had suddenly arisen, as if out of the ground, a new political Germany which rivalled France in strength.

The thing to do was to recover his waning prestige by a victory over Prussia.
The Empress Eugenie, devoutly Catholic in her sympathies, saw, in the ascendancy of Protestant Prussia and the humiliation of Catholic Austria, an impious blow aimed at the Catholic faith in Europe.

So, as the emperor wanted war, and the empress wanted it, it only remained to make France want it too; for war it was to be.
Only one obstacle existed: there was nothing to fight about! But that was overcome.

In 1870 the heart of the people of France was fired by the news that the French Ambassador had been publicly insulted by the kindly old King William.

There had been some diplomatic friction over the proposed occupancy of a vacant throne in Spain by a member of the Hohenzollern (Prussian) family.
Whether true or false, the rumor served the desired purpose.