Kosovo is a landlocked, partially recognized country in the Western Balkans. It is surrounded by mountains, including the Sar Mountains in the south and southwest and the Kopaonik Mountains in the north. In the center of the country, the Metohija and Kosovo plains provide some of Kosovo's most fertile lands. Kosovo's economic and political core is Pristina, the country's capital and most populated city. Kosovo is one of the smallest countries in Europe, with a population of roughly 2 million people. In the Middle Ages, Kosovo's territory was the heart of the Serbian medieval state and was the seat of the Serbian Orthodox Church. But the territory fell under Ottoman control in the 15th century, a situation that lasted for five centuries. After World War II, Kosovo became an autonomous province within Yugoslavia. The 20th century saw frictions between ethnic Serbian and Albanian communities in Kosovo, which peaked during the Kosovo War of 1998 and 1999. The war only ended after the intervention of the United Nations and NATO. Kosovo unilaterally declared independence from Serbia in 2008. While the country is recognized by more than a hundred members of the United Nations, including the United States and most members of the European Union, several countries such as Russia and China do not recognize it as a sovereign state. Under EU sponsorship, Serbia and Kosovo are negotiating a normalization of their relations, but significant problems remain, including the status of areas with significant Serbian populations. Kosovo has introduced some reforms since independence, but it still has weak economic and political institutions and a large informal sector, while remittances and foreign aid remain important for the economy. Kosovo's partial recognition is a significant obstacle in the country's bid to join international organizations such as the United Nations and the European Union and to better integrate into the global economy. Its main challenge is therefore to gain full international recognition and to reach some degree of stability after decades of turbulence.