Why did France declare war on Prussia in July 1870?

France declared war on Prussia on July 19, 1870, primarily due to a diplomatic crisis triggered by the candidacy of Prince Leopold of Hohenzollern-Sigmaringen for the vacant Spanish throne. France feared that a Prussian-aligned monarch in Spain would threaten French security and influence.12

Although Leopold withdrew his candidacy under French pressure, the French government demanded further assurances from the Prussian King William I that the candidacy would never be renewed. When the King refused this demand, the Prussian chancellor, Otto von Bismarck, edited and published a report of the encounter—known as the Ems telegram—in a manner designed to insult both the French and the Prussians.13

This provocation successfully incited public demand for war in both nations. While the telegram provided the immediate excuse, the underlying cause was France's determination to check Prussia's rising power and Napoleon III's desire to restore his declining domestic popularity through a military victory.132