Croatia, officially the Republic of Croatia, is a sovereign state between Central Europe, Southeast Europe, and the Mediterranean. Its capital city is Zagreb, which forms one of the country's primary subdivisions, along with its twenty counties. Croatia covers 56594km2 and has diverse, mostly continental and Mediterranean climates. Croatia's Adriatic Sea coast contains more than a thousand islands. The country's population is 4.28 million, most of whom are Croats, with the most common religious denomination being Roman Catholicism.The Croats arrived in the area of present-day Croatia during the early part of the 7th century AD. They organised the state into two duchies by the 9th century. Tomislav became the first king by 925, elevating Croatia to the status of a kingdom. The Kingdom of Croatia retained its sovereignty for nearly two centuries, reaching its peak during the rule of Kings Petar Krešimir IV and Dmitar Zvonimir. Croatia entered a personal union with Hungary in 1102. In 1527, faced with Ottoman conquest, the Croatian Parliament elected Ferdinand I of the House of Habsburg to the Croatian throne. In 1918, after World War I, Croatia was included in the unrecognized State of Slovenes, Croats and Serbs which seceded from Austria-Hungary and merged into the Kingdom of Yugoslavia. The fascist Croatian puppet state backed by Fascist Italy and Nazi Germany existed during World War II. After the war, Croatia became a founding member and a federal constituent of the Socialist Federal Republic of Yugoslavia, a constitutionally socialist state. On 25 June 1991 Croatia declared independence, which came wholly into effect on 8 October of the same year. The Croatian War of Independence was fought successfully during the four years following the declaration.