c) Arizona and New Mexico

The Gadsden Purchase of 1853 achieved the acquisition of land that now makes up southern Arizona and New Mexico. This transaction between the United States and Mexico involved the U.S. paying $10 million for about 30,000 square miles of territory. The purchase was motivated by the desire to build a southern transcontinental railroad and to settle boundary disputes following the Mexican-American War. The Gadsden Purchase completed the continental outline of the United States and played a crucial role in the expansion of the U.S. rail network.