The Gadsden purchase

In July 1853 President Franklin Pierce instructed James Gadsden, minister to Mexico, to make a treaty not only settling the issues involved but also securing enough territory for the proposed southern railroad route. Financial needs of the administration of Antonio López de Santa Anna aided negotiation of a treaty whereby territory in northern Mexico was sold to the United States. The Gadsden Treaty, as it became known, abrogated Article XI of the Treaty of Guadalupe Hidalgo, but the United States was to aid in suppressing Indian depredations. For these concessions the United States would pay Mexico $15 million and assume all claims of its citizens against Mexico, including the Hargous claim. The United States promised to cooperate in suppressing filibustering expeditions.

The treaty met strong opposition in the Senate, where antislavery senators condemned further acquisition of slave territory. Lobbying by speculators worsened the treaty's reputation. Some senators objected to furnishing Santa Anna financial assistance. The Senate, by a narrow margin, ratified the treaty on 25 April 1854, but only after reducing the territory to be acquired to that considered essential for the railroad route. The Senate also deleted all mention of private claims and filibustering expeditions. The payment to Mexico was lowered to $10 million, and the Senate inserted an article promising American protection to the Sloo grantees. A combination of the advocates of the southern railroad route and the friends of the Sloo grant made ratification possible.

By the Gadsden Treaty the United States secured 45,535 square miles of territory. This tract became known as the Gadsden Purchase and today encompasses the southern part of Arizona and New Mexico.