Prussia's behavior during the Polish insurrection brought her into a close friendship with Russia. The result was seen when Austria and Prussia, in 1864, invaded the German provinces of Denmark, when Russia prevented intervention, and Denmark lost the two provinces by the Treaty of Vienna, October 30, 1864. Soon after Prussia and Austria quarreled about the spoils. The countries of South Germany supported Austria. War began on June 18, 1866, and little over two months later, on August 23, 1866, it ended by the Peace of Prague, which gave to Prussia Hanover, Schleswig-Holstein, Hesse, Nassau, and the city of Frankfort. Prussia did not annex Wurtemburg in compliment to the czar, who was related to its king by marriage. If Russia looked carelessly upon Prussia's growth, not so Napoleon III of France. He saw in it a threat, and to offset Prussia's increase of power, tried to secure other territory. It was evident that nothing but a pretext was needed to bring on war. It was found, and Napoleon declared war on July 15, 1870. Once again it was Alexander who protected Prussia on the east, by threatening Austria which would gladly have seized the opportunity to avenge 1866. As a consequence France (p. 230) had to fight the whole of Germany; and Russia seized the opportunity for repudiating the treaty of Paris of 1856, which forbade the construction of arsenals on the coast of the Black Sea and did not permit any war vessels in it. None of the powers felt any inclination to fight Russia single-handed, but Prussia proposed a conference, which was held at London. The result was that Russia was left free in the Black Sea, but the sultan has the right to close the Dardanelles to warships. On January 18, 1871, the King of Prussia became German Emperor, and in the following year the Emperor of Russia, the Emperor of Austria, and the German Emperor met at Vienna, with the result that an alliance was concluded among the three powers.