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How did the Hohenzollern candidacy for the Spanish throne lead to war between France and Prussia?

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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 candidacy of Prince Leopold of Hohenzollern-Sigmaringen for the Spanish throne triggered a diplomatic crisis that led to the Franco-Prussian War in 1870.12

When it was revealed that a relative of the Prussian king was a candidate for the Spanish throne, the French government, led by Napoleon III, feared that Prussian influence would threaten French security. Although Leopold withdrew his candidacy on July 12, the French government demanded that King William I provide a formal assurance that the candidacy would never be renewed.234

The King politely refused this demand during a meeting with the French ambassador at Ems. Chancellor Otto von Bismarck subsequently edited the report of this encounter—the Ems telegram—to make it appear that both parties had insulted one another. The publication of this version inflamed public opinion in both nations, providing the catalyst for France to declare war on July 19, 1870. Ultimately, the conflict was driven by French efforts to check Prussian expansion and restore the prestige of the French Empire.24