France declared war in the Prussian dominated north German confederation because

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Because of the revelation of the Ems Dispatch, a document that reveals a supposed exchange of insults between the Prussian king and the French ambassador. The effect was inflamatory in both countries. But this was only a pretext. Tensions and historic grievances - Prussia was an enemy of France in the Napoleonic wars - had accumulated for several decades and there were the always pressing issues of the balance of power in Europe. A dispute over the royal Spanish succession also accelerated the Franco-Prussian War that finally erupted in 1870.