 Complete the following phrase:  Ever since independence in 1821, Mexico City remains the countrys largest and most important city. Post-independence, U.S. forces captured Mexico City during the MexicanAmerican 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 citys population stood at about 500,000. The citys 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 