Just as Franklin Pierce came to power in Washington, Santa Anna came back into power in Mexico City. He needed money to reform the Mexican army, what with all the Americans who saw the border as nothing more than a suggestion or temptation to filibuster. After Texas, the Mexican War, and William Walker’s attempt to seize Lower California and Sonora, anybody could see the pattern. Americans would cross the border, theoretically leaving the United States behind, but then find a way to bring it to them. Further complicating matters, while the Treaty of Guadalupe-Hidalgo obligated the United States to prevent Indian raids across the border into Mexican territory, the US did very little to that end and insisted that the treaty required no more than the same effort that Mexico expended on the same problem. The US lacked men, money, and infrastructure to police the entire border. It also refused to compensate Mexicans for their losses due to its inability.
On top of all that, Mexico and the United States disagreed on where the border actually ran. The treaty commissioned a binational survey team to plot it all out, which they did. There the problems began. The treaty drew a line from El Paso, but that line went on an attached map more than twenty years out of date. The surveyors learned that El Paso, actually rested some distance to the southwest of the old man. Did the true line, which ran from eight miles north of El Paso, follow the map copy or the results of the survey? Mexico naturally preferred the map, which left it with more land. The United States preferred the survey for just the same reason. A few thousand square miles of desert and a few thousand people might not seem like a lot to have an international dispute over, but who took responsibility for keeping order there? What would happen if the Mexican army and United States army arrived at the same place to prosecute their claims? An engineered incident just like that started the Mexican War. The disputed land also included the Mesilla Valley, a relatively straight, flat piece of land perfect for a railroad.
New Mexico’s territorial governor tried to resolve the dispute on his own, proclaiming the land part of his domain. That exceeded Pierce’s tolerance and he replaced the governor with another. But he sent, or rather ratified Jefferson Davis’ sending, of James Gadsden to negotiate a new boundary settlement with an eye toward the Mesilla Valley and other land for a railroad to the Gulf of California. That other land included all of Lower California, which had also been in the instructions James K. Polk sent with Nicholas Trist after the Mexican War. To that Pierce added sections of Tamaulipas, Nuevo Leon, Coahuila, Chihuahua, and Sonora. The entire border would move southward, save for a small section that would still follow the Rio Grande in along west Texas, dramatically so in the case Lower California and at the Gulf of Mexico. In exchange for all that land, with its railroad route and known and suspected mineral assets, and concessions to build a canal or railroad over the isthmus of Tehuantepec, the United States would give Santa Anna $50,000,000, the equivalent of $1,359,509,422.48 in 2012.