The Aztec civilization began in the 12th-13th century in the Valley of Mexico. They arrived as a small, nomadic group but built a large empire through conquest. The Aztecs developed advanced cities with infrastructure like Tenochtitlan, which had palaces, temples, markets and canals. They also invented cultural elements like the calendar, popcorn, and the passion flower. The Aztec empire declined when the Spanish led by Hernan Cortez invaded in the 15th century and defeated the Aztecs, whose weapons were inferior.