Ever since independence in 1821, Mexico City remains the country's largest and most important city. Post-independence, U.S. forces captured Mexico City during the Mexican–American War, and the city saw violence during the Reform War and the French Intervention as well as the Mexican Revolution. At the beginning of the 20th century, the city's population stood at about 500,000. The city's history in the 20th and 21st centuries has been marked by explosive population growth and its accompanying problems. The city center deteriorated. The government has had problems keeping up with basic services, but the building of the Mexico City Metro has alleviated some major transportation problems. Smog became a serious problem as the shanty towns evolved, formed by the poor of the country migrating to the city. Since the 1985 Mexico City earthquake, which caused significant damage to the center of the city, efforts have been made to correct some of these problems. In the 2000s, businessman and philanthropist Carlos Slim created a foundation to revitalize the historic center as well as sites near the Basilica of the Virgin of Guadalupe.