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On 19 July 1870, the French Empire declared war on the Kingdom of Prussia. The southern German states took the side of Prussia in accordance with their defensive alliances and victories in August and September 1870, over the French armies led to the willingness of the Southern German princes to join the North German Confederation.
On 9 and 10 December 1870, the Reichstag voted to offer the Emperor's title to the Prussian king. In addition, the country was to be renamed "German Reich". This became effective on 1 January 1871 with a new constitution.