Europe is, conventionally, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, and by the Caucasus Mountains to the southeast. Europe is washed upon to the north by the Arctic Ocean, to the west by the Atlantic Ocean, to the south by the Mediterranean Sea, and to the southeast by the Black Sea and the waterways connecting it to the Mediterranean. Yet, the borders for Europe—a concept dating back to classical antiquity—are somewhat arbitrary, as the term continent can refer to a cultural and political distinction or a physiographic one. Europe is the world's second-smallest continent by surface area, covering about 10,180,000 square kilometres (3,930,000 sq mi) or 2% of the Earth's surface and about 6.8% of its land area. Of Europe's approximately 50 states, Russia is the largest by both area and population, while the Vatican City is the smallest. Europe is the third most populous continent after Asia and Africa, with a population of 731,000,000 or about 11% of the world's population; however, according to the United Nations, Europe's share will fall to 7% in 2050, numbering 653,000,000. Europe is the birthplace of Western culture. European nations played a predominant role in global affairs from the 16th century onwards, especially after the beginning of Colonialism. By the 17th and 18th centuries, European nations controlled most of Africa, the Americas, and large portions of Asia. World War I and World War II led to a decline in European dominance in world affairs as the United States and Soviet Union took prominence. The Cold War between those two superpowers divided Europe along the Iron Curtain. European integration led to the formation of the Council of Europe and the European Union in Western Europe, both of which have been expanding eastward since the fall of the Soviet Union in 1991.