Spain is located in southwestern Europe on the Iberian Peninsula and is bordered by the Mediterranean Sea, France, the Atlantic Ocean and Portugal. Spain is separated from France by the Pyrenees, a protective mountain barrier between the two countries. The Strait of Gibraltar divides Spain from Africa, which at its narrowest is about 15 kilometers (9 miles) wide. This eased the invasion of the Moors in the eighth century during the rise of the Islamic caliphate. The Arab presence lasted until the end of the 15th century, when Christian Spanish kingdoms unified modern Spain. This unification helped Spain compete with other maritime powers such as the United Kingdom and continental powers like France. Spain’s access to the Mediterranean and the North Atlantic, along with its need for natural resources, promoted its consolidation into one of the greatest naval and colonial powers of Europe. While Spain has had external challenges, its main geographic challenge comes from within. Spain’s core is Madrid, the country’s capital, most populated city and political center. Madrid sits on Spain’s Meseta Central and was chosen as the capital in the 16th century to allow more centralized control of the country. But Spain’s mountainous terrain has historically determined its political life by hindering communication between different regions within the country. This geography has led to the emergence of regionalist and separatist movements, especially in Catalonia and the Basque Country. Spain’s challenge, then, is to bring about a truly united nation with a balance of power between the central government in Madrid and the autonomous regions, especially as Catalonia.