In 1446 Tenochtitlan was badly flooded, so Montezuma I ordered a great embankment to be built to protect the city. Then, several years of bad harvests caused thousands of people to die of starvation. Pledging to never let this happen again, the Emperor set out to expand outside of the valley, conquering wealthy lands with good farmland. Two years later, the Aztecs formed an alliance with two neighboring cities, Texcoco and Tlacopan. This "Triple Alliance" continued to conquer other lands, and by the 16th century they controlled an empire of four million people.