Final answer:
The Mexican Cession was caused by border disputes following Texas's annexation and the US's expansionist policy, leading to the Mexican-American War and the acquisition of a large part of Mexico's territory.
Step-by-step explanation:
The cause of the Mexican Cession was primarily due to border disputes between the United States and Mexico following the annexation of Texas. Tensions over the boundary escalated, with the U.S. claiming the Rio Grande as the border while Mexico insisted on the Nueces River. Beyond mere border disputes, this tension also reflected the expansionistic desires of the United States, particularly under President Polk, who sought to extend American territory significantly to the west, a concept known as Manifest Destiny. Strategic moves such as John Slidell's mission to Mexico were attempts to purchase substantial portions of Mexican territory. The failure of diplomacy eventually led to the outbreak of the Mexican-American War, which resulted in the U.S. gaining a vast amount of land from Mexico and heightened issues of violence and racial antagonism.