The Mexican Cession of 1848 is a historical name in the United States for the region of the modern day southwestern United States that Mexico ceded to the U.S. in the Treaty of Guadalupe Hidalgo in 1848, it had not been part of the areas east of the Rio Grande which had been claimed by the Republic of Texas, though the Texas annexation resolution in which year,incorporation of the Republic of Texas into the United States of America, which was admitted to the Union as the 28th state, had not specified the southern and western boundary of Texas?