The US purchase of about 29,640 square miles of land in 1853 from Mexico. Today the area is the southern quarter of Arizona and a small strip of New Mexico. It is bounded on the west by the Colorado River, the east by the Rio Grande, and the north by the Gila River.

The Gadsden Purchase was necessary due to a misunderstanding in the 1848 Treaty of Guadalupe Hidalgo, which had defined the border between the United States and Mexico on an innacurate map. Ah, those crazy 19th Century Treaties!

The government of Mexico, under the leadership of Antonio Lopez de Santa Anna was in severe need of finances, which eased the path for the treaty. The U.S. Senate ratified the treaty on April 25th, 1854 after a heated debate on the adding of more slave territory to the US. The Senate reduced the size of the land slightly, and lowered the purchase to $10 million from its original $15 million price.

The treaty was not well received in Mexico, and helped lead to the downfall of Santa Anna.