The United States buys approximately seventy-seven thousand…

The United States buys approximately seventy-seven thousand square kilometers (thirty thousand square miles) of land from Mexico, to facilitate railroad building in the Southwest, on December 30, 1853.

By the Treaty of Mesilla that is to take effect on June 8, 1854, the Gadsden Purchase refers to the region of present-day southern Arizona and southwestern New Mexico that the United States acquires from Mexico.

The purchase includes lands south of the Gila River and west of the Rio Grande that the U.S. needs to build a transcontinental railroad along a deep southern route, which the Southern Pacific Railroad will later complete in 1881–1883.

The purchase also aims to resolve other border issues.

The first draft is signed on December 30, 1853, by James Gadsden, U.S. ambassador to Mexico, and by Antonio López de Santa Anna, president of Mexico.

The U.S. Senate will vote in favor of ratifying it with amendments on April 25, 1854, and will then transmit it to President Franklin Pierce.

Mexico's government and its General Congress or Congress of the Union will take final approval action on June 8, 1854, when the treaty takes effect.

The purchase is the last substantial territorial acquisition in the contiguous United States, and defines the Mexico–United States border.

The financially-strapped government of Santa Anna has agreed to the sale, which will net Mexico ten million dollars (equivalent to two hundred and twenty million dollars in 2016).

After the devastating loss of Mexican territory to the U.S. in the Mexican–American War (1846–48) and the continued filibustering by U.S. citizens, Santa Anna may have calculated it was better to yield territory by treaty and receive payment rather than have the territory simply seized by the U.S.