Augustenborg, was more controversial. The separation of the two duchies was challenged by the Augustenborg heir, who claimed, as in 1848, to be rightful heir of both Schleswig and Holstein. The promulgation of a common constitution for Denmark and Schleswig in November 1863 prompted Otto von Bismarck to intervene and Prussia and Austria declared war on Denmark. This was the Second War of Schleswig, which ended in Danish defeat. British attempts to mediate in the London Conference of 1864 failed, and Denmark lost Schleswig (Northern and Southern Schleswig), Holstein, and Lauenburg to Prussia and Austria. Province of Prussia Contrary to the hopes of German Schleswig-Holsteiners, the area did not gain its independence, but was annexed as a province of Prussia in 1867. Also following the Austro-Prussian War in 1866, section five of the Peace of Prague stipulated that the people of Northern Schleswig would be consulted in a referendum on whether to remain under Prussian rule or return to Danish rule. This condition, however, was never fulfilled by Prussia. During the decades of Prussian rule within the German Empire, authorities attempted a Germanisation policy in the northern part of Schleswig, which remained predominantly Danish. The period also meant increased industrialisation of Schleswig-Holstein and the use of Kiel and Flensburg as important Imperial German Navy locations. The northernmost part and west coast of the province saw a wave of emigration to America, while some Danes of North Schleswig emigrated to Denmark. Plebiscite in 1920 Following the defeat of Germany in World War I, the Allied powers arranged a plebiscite in northern and central Schleswig. The plebiscite was conducted under the auspices of an international commission which designated two voting zones to cover the northern and south-central parts of Schleswig. Steps were taken to also create a third zone covering a southern area, but zone III was cancelled again and never voted, as the Danish government asked the commission not to expand the plebiscite to this area. In zone I covering Northern Schleswig (10 February 1920), 75% voted for reunification with Denmark and 25% voted for Germany. In zone II covering central Schleswig (14 March 1920), the results were reversed; 80% voted for Germany and just 20% for Denmark. Only minor areas on the island of Föhr showed a Danish majority, and the rest of the Danish vote was primarily in the town of Flensburg. On 15 June 1920, Northern Schleswig officially