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What caused the Franco-German War of 1870?

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Portrait of Napoleon III in a dark military uniform with gold epaulettes, a red sash, and medals, set against a deep red background. Painted by Hippolyte Flandrin. — Napoleon III Napoleon III, detail of a portrait by Hippolyte Flandrin, 1862; in the Versailles Museum.

The Franco-German War of 1870 was driven by a complex mix of long-standing geopolitical tensions and an immediate diplomatic crisis.

Underlying Causes

The root cause was France's determination to check Prussia’s rising power following its victory over Austria in 1866, which threatened French hegemony in Europe. Simultaneously, Napoleon III sought to restore his declining domestic popularity and the prestige of his empire through a successful foreign policy.12

The Immediate Trigger

The conflict was precipitated by the candidacy of Prince Leopold of Hohenzollern-Sigmaringen for the vacant Spanish throne. France feared that a Hohenzollern monarch in Spain would result in a strategic encirclement by Prussia. Although Leopold withdrew his candidacy, France demanded that King William I promise that no Hohenzollern would ever seek the throne again.213

When the King refused this demand, Otto von Bismarck edited the Ems telegram—a report of the meeting—to make it appear that the King and the French ambassador had insulted one another. This provocation successfully goaded the French government into declaring war on July 19, 1870.231