separate pieces of land for a combined area of making it the second largest Native American land holding in the United States. The lands include, the main reservation, the Gila Bend Reservation, San Xavier Reservation, and Florence Village. Of the four lands bases, the largest is the main reservation at more than . The San Xavier reservation is the second largest land base, and contains just south of Tucson. The Gila Bend Indian Reservation is and Florence Village . With the 1853 Gadsden Purchase, the territory of the Tohono Oʼodham was split between the United States and Mexico. Consequently, the Nation is directly exposed to the Mexico–United States border for . There is no reservation established for the Tohono Oʼodham people in Mexico, thus the Nation's southern border is the Mexico–United States border. Border issues Pre-contact to 1900 Prior to colonization, the O'odham migrated along a north–south axis in a "two village" system, rotating between summer and winter settlements. These migrations formed the foundation of their subsistence economies and enabled religious pilgrimages. This pattern continued throughout Apache, Spanish, and American expansion, but shifted with the re-drawing of boundaries that followed the Mexican–American War. At the Border of Empires: The Tohono O'odham, Gender, and Assimilation, 1880–1934. Tucson, AZ: University of Arizona Press, 2013. Unlike aboriginal groups along the U.S.-Canada border, the Tohono Oʼodham were not offered dual citizenship when the US drew a border across their lands in 1853 by the Gadsden Purchase. The Treaty of Guadalupe Hidalgo did not specify the rights of the Oʼodham in crossing the international border. The population was split between Mexico and the United States, however, after the treaty the U.S. government guaranteed that the Oʼodham freedom of movement would be protected. For decades, members of the nation continued to move freely across the current international boundary. Throughout this time, tribal members traveled and migrated to work, participate in religious ceremonies, keep medical appointments in Sells, Arizona and visit relatives. The O'odham were deliberate in attending their religious festivals, and they would leave their employers for two to four weeks to travel to Magdalena, Sonora. Oʼodham labor was so valued that employers began to drive their O'odham employees to the festivals rather than lose 4–8 days of labor while tribal members traveled the 140–200 miles by wagon. The end of the 19th century and beginning of the 20th saw a decline in the subsistence economies