But the treaty did not complete American border expansion to the southwest. In the first place, the Treaty of Guadalupe Hidalgo, like the one after the American Revolution, was based on an inaccurate prewar map, so a boundary commission had to run the whole line on the ground, a process causing new quarrels with Mexican state and federal governments. Second, Mexicans complained that Indians north of the new border were increasing their raids into Mexico. They demanded that the United States carry out Article 11 of the peace treaty, requiring it to restrain the border Indians from raiding—an impossibility with the available infantry troops. Third, American filibusters crossed the border almost at will, seeking to detach Mexican states and complete the absorption process begun by the war. Fourth, American railroad interests discovered belatedly that the best route for a southern transcontinental railroad lay south of the Gila River in Mexican territory. Finally the question of rail transit across the Isthmus of Tehuantepec, omitted from the peace treaty, was taken up by the commercially minded Taylor-Filmore administration, as Americans acquired interests in transit concessions.
All of these considerations figured in the effort of the Franklin Pierce administration to acquire more land from Mexico. Pierce appointed the railroad owner James Gadsden minister to Mexico and gave him a list of desired territories and corresponding prices to be offered. The maximum price was for Lower California and a good part of northern Mexico, the minimum for a strip of territory south of the Gila River big enough for the contemplated transcontinental railroad.
In Mexico, Gadsden's negotiating partner was Antonio López de Santa Anna, the opponent of the Americans through the Texas question and the Mexican War. Santa Anna was as eager as ever for money but wary of nationalist opposition to more land grants. Gadsden was a bumbling diplomat given to grandiloquent and imprecise language. Other complications were efforts by another agent from Pierce to insert the Tehuantepec question and a filibuster expedition to Lower California by the notorious guerrilla leader William Walker. Santa Anna, now desperate, put out feelers for British or French aid to no avail. In the end, he gave up a thin triangle of land south of the Gila large enough for a railroad and released the United States from Article 11 of the peace treaty. For this minimal grant, the United States agreed to pay $15 million plus $5 million for private American claims. The Senate was in the midst of a furious debate over the Kansas-Nebraska Act but took up the Gadsden treaty, too. It lowered the purchase price to $10 million and somewhat increased the land area acquired. Outside the continent, it opened the Isthmus of Tehuantepec to American transit, but it did not give the United States access to the Gulf of California, as some had hoped.