The western states of New Mexico, Arizona and Utah came into being first as territories of the United States by acts passed by Congress and signed by the President in the early 1850s. Their borders were dictated basically by conflicts with Mexico and the looming civil war in the United States.
There were factions in the U.S. government that wanted more of Mexican territory ceded to the United States following the war with Mexico that brought into the union the State of Texas. At that time, Mexico claimed all the land below the Gila River, including the villages of Tucson and Casa Grande, but the Treaty of Guadalupe Hidalgo changed those claims. However, that agreement was ambiguous and didn’t define boundaries very well.
Texas was well outlined, but New Mexico (a new territory) was loosely outlined on its western borders to include all the lands that now lie within Arizona, and some of Utah and Nevada.
The U.S. Congress was also divided. Members from the northern states insisted that the territories be restricted from allowing slavery to be practiced in the western lands while the southern representatives demanded that since they were “southern” states that those restrictions should not be included.
To add to the confusion, many members of the national government and influential businessmen (especially railroad tycoons) wanted more northern Mexico lands in order that a pacific port could be established on the Gulf of California and enough land be ceded to America so that railroad lines could be constructed near the northern banks of the Gulf of California. This meant that the Baja California peninsula would also become part of the United States. Mexico, of course, balked at this idea.
In 1853 James Gadsden went to Mexico City as United States Envoy. He had instructions from the President to more accurately define our southern border. He arrived in the city at a fortuitous time. This was the height of the last dictatorship of Antonio Lopez de Santa Anna. He had returned to Mexico as its ruler that spring. Gadsden realized that Santa Anna needed funds for his government and Gadsden negotiated and signed a treated with the new government on December 30, 1853. This document, called the Gadsden Treaty, more clearly defined the border between Mexico and the United States and ceded much of the current state of Arizona to the U.S.
Mexican citizens denounced the agreement and revolted against Santa Anna, eventually driving him from power. The American congress also denounced it on both sides. Thomas Hart Benton said the new territory was “desolate, desert, and God-forsaken.” Kit Carson said, “a wolf could not make a living upon it!” But the treaty was finally ratified on April 21, 1854 and signed by President Franklin Pierce.
Since the treaty called for a narrow band of land at the head of the Gulf of California to stay with Mexico and the Baja to remain as Mexican land, the southern border of what is now Arizona has an angular line that was extremely difficult to map.
The treaty called for a sum of fifteen million dollars to be paid to Mexico, but the Senate reduced this amount to ten million. The money was paid so that the southern section of Arizona would be added to the territory. Since the whole Louisiana Purchase had cost only six million and had included all of the drainage of the Missouri River plus the northwest coastal section of the United States, the $10-million price for southern Arizona seemed extreme at the time.
There will always be problems with the southern border of Arizona, but there’s little question now that the purchase price was a bargain.