The Aztecs soon controlled the riches of the Valley of Mexico, an important trade center, and by the end of the 15th century Tenochtitlan was a beautiful city of fountains, gardens and canals occupying the small islands that dotted the lake. Eventual land reclamation resulted in the creation of one large island, connected to the mainland by causeways. Tenochtitlan's population was about 300,000—possibly the world's largest city at its time. Then the Spanish arrived.
On Nov. 8, 1519, explorer Hernando Cortés became the first white man to enter Tenochtitlan's ceremonial center, today's Zócalo. Moctezuma II, the Aztec emperor, met Cortés with rich gifts and offered no resistance to his entry into the city. He believed the Spaniard to be a divine envoy of Quetzalcóatl, the fair-skinned, golden-haired god of civilization who according to legend was to return in the year of One Reed (Ce Acatl). On the Aztec calendar, 1519 was that year.
This case of mistaken identity brought about Moctezuma's downfall. Taking the ruler captive, Cortés and his troops remained in Tenochtitlan. After a long siege, a once-mighty city collapsed on Aug. 13, 1521. The Spaniards built their own city atop the ruins of the capital, leaving the outer periphery to the vanquished. Aztecs gradually intermingled with the Spanish, resulting in mestizos, persons of mixed Spanish and Indian blood who comprise the great majority of Mexico's present-day population.
Mexico City remained under Spanish rule for exactly 3 centuries, culminating in the decade-long fight for independence that followed Grito de Dolores, Father Miguel Hidalgo's impassioned speech advocating Mexican freedom, in 1810. It was finally taken by an army of patriots under Gen. Agustín de Iturbide, who entered the city on Sept. 27, 1821. Iturbide appointed himself emperor of the new nation in 1822 and was crowned in Mexico City as Agustín I, but his power was short lived; in December 1822 the republic was proclaimed and Iturbide was forced to abdicate.
During the 1860s the first colonias, or residential districts, began to appear. Modernization began on a large scale under the reign of dictator Porfirio Díaz from 1876 to 1910. Mexico City benefited from the establishment of such amenities as electric lighting, streetcars and a drainage system. The Palace of Fine Arts (Palacio de Bellas Artes) and other monumental public buildings were constructed, their design modeled after prevailing European neoclassic styles.
Modernization continued after adoption of the Constitution of 1917, bringing a steady stream of impoverished mestizos and Indians from the countryside. They crowded into working-class colonias, while luxurious residential districts housed the wealthy few. Skyscrapers began to define the city's skyline in the 1930s. The first sections of a modern subway system were completed in 1971; today Metro provides inexpensive public transportation to millions of people daily.
In the last half of the 20th century the metropolitan area expanded in all directions, absorbing formerly separate towns like Churubusco, Coyoacán, Iztapalapa, San Angel, Tlalpan, Villa de Guadalupe and Xochimilco. Today, greater Mexico City encompasses hundreds of colonias (residential neighborhoods).