Question: 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?

Context paragraphs:
Juan Cortina: Juan Nepomuceno Cortina Goseacochea (May 16, 1824 – October 30, 1894), also known by his nicknames Cheno Cortina, the Red Robber of the Rio Grande and the Rio Grande Robin Hood, was a Mexican rancher, politician, military leader, outlaw and folk hero.  He was an important caudillo, military general and regional leader, who effectively controlled the Mexican state of Tamaulipas as governor.  In borderlands history he is known for leading a paramilitary mounted Mexican Militia in the failed Cortina Wars.  The "Wars" were raids targeting Anglo-American civilians whose settlement Cortina opposed near the several leagues of land granted to his wealthy family on both sides of the Rio Grande.  Anglo families began immigrating to the Lower Rio Grande Valley after the Mexican Army was defeated by the Anglo-Mexican rebels of the Mexican State of Tejas, in the Texas Revolution.  From 1836 to 1848 when Cortina was 12–24 years old, parts of the Cortina Grant North of the Rio Grande River was in the disputed territory between the Rio Grande and the Nueces Rivers, claimed by both Mexico and the Republic of Texas.  The situation had a big impact on Cortina and his perspective on government and power.  When the United States defeated Mexico in the Mexican-American War in 1848, Mexico was forced to concede the disputed territory to Texas.  Cortina opposed this concession.  However, Cortina's Mexican militia was easily defeated and forced to flee into Mexico when the Texas Rangers, the United States Army and the local militia of Brownsville, Texas and Matamoros, Tamaulipas.  According to Robert Elman, author of "Badmen of the West", Cortina was the first "socially motivated border bandit," similar to Catarino Garza and Pancho Villa of later generations.  His followers were known as the "Cortinistas."

Mexican Cession: 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 two years earlier had not specified the southern and western boundary of Texas.  The Mexican Cession (529,000 sq. miles) was the third largest acquisition of territory in US history.  The largest was the Louisiana Purchase, with some 827,000 sq. miles, followed by the acquisition of Alaska (about 586,000 sq. miles).

Reconquista (Mexico): The Reconquista ("reconquest") is a term that is used (not exclusively) to describe the vision by different individuals, groups, and/or nations that the U.S. Southwest should be politically or culturally conquered by Mexico.  These opinions are often formed on the basis that those territories had been claimed by Spain for centuries and had been claimed by Mexico from 1821 until being ceded to the United States in the Texas annexation (1845) and the Mexican Cession (1848), as a consequence of the Mexican American War.

Treaty of Guadalupe Hidalgo: The Treaty of Guadalupe Hidalgo ("Tratado de Guadalupe Hidalgo" in Spanish), officially entitled the Treaty of Peace, Friendship, Limits and Settlement between the United States of America and the Mexican Republic, is the peace treaty signed on February 2, 1848, in the Villa de Guadalupe Hidalgo (now a neighborhood of Mexico City) between the United States and Mexico that ended the Mexican–American War (1846–48).  The treaty came into force on July 4, 1848.

United States Boundary Commissioner: Article V of the TREATY OF PEACE, FRIENDSHIP, LIMITS, AND SETTLEMENT BETWEEN THE UNITED STATES OF AMERICA AND THE UNITED MEXICAN STATES CONCLUDED AT GUADALUPE HIDALGO (known commonly as the Treaty of Guadalupe Hidalgo ) declared that the new boundary between the two republics of the United States and Mexico would lay "three leagues from land, opposite the mouth of the Rio Grande, otherwise called Rio Bravo del Norte, or Opposite the mouth of its deepest branch, if it should have more than one branch emptying directly into the sea; from thence up the middle of that river, following the deepest channel, where it has more than one, to the point where it strikes the southern boundary of New Mexico; thence, westwardly, along the whole southern boundary of New Mexico (which runs north of the town called Paso) to its western termination; thence, northward, along the western line of New Mexico, until it intersects the first branch of the river Gila; (or if it should not intersect any branch of that river, then to the point on the said line nearest to such branch, and thence in a direct line to the same); thence down the middle of the said branch and of the said river, until it empties into the Rio Colorado; thence across the Rio Colorado, following the division line between Upper and Lower California, to the Pacific Ocean. "  It delineated the boundary through its text but wasn't written with much knowledge of the area's actually geography; A commissioner and surveyor from each country were appointed to precisely locate landmarks and accurately map the new border, relying on the treaty as their guide.

Horcón Tract: The Horcón Tract is a small tract of land (a "banco") that, prior to 1905, was situated north of the Rio Grande in Texas.  As the Rio Grande formed the international border, the tract was unambiguously American territory.  In July 1906, however, the American Rio Grande Land and Irrigation Company dug a cutoff to shorten the course of the river, thus bypassing the tract.  As a result, the 461 acre of the tract (including the former riverbed) became located south of the river.  The company was eventually taken to court and fined for the diversion of the river, and the land was agreed to remain American territory, in accordance with the border treaty.  In 1929 the town of Río Rico was founded near the tract on the Mexican side of the Rio Grande.  As the Rio Grande changed its course after floods, the settlement was progressively pushed into the tract.  The residents, being mostly of Mexican heritage, accepted the authority of the Mexican government and all parties generally acted as if the tract was Mexican territory.  However, the United States never actually relinquished the land, and the issue was re-discovered decades later by a researcher.  The Boundary Treaty of 1970 provided for the Horcón Tract to revert to Mexico upon the completion of two new flood control projects.  Thus in 1977 the United States officially ceded the tract of land to Mexico.  The Board of Immigration Appeals later determined that persons born in the Horcón Tract between 1906 and 1972 could not be deported, effectively if not technically granting citizenship.  As a result, a large portion of the population of Rio Rico moved to the United States.

Botiller v. Dominguez: Botiller v. Dominguez, 130 U.S. 238 (1889), was a decision by the United States Supreme Court dealing with the validity of Spanish or Mexican land grants in the Mexican Cession, the region of the present day southwestern United States that was ceded to the U.S. by Mexico in 1848 under the Treaty of Guadalupe Hidalgo.

Texas annexation: The Texas annexation was the 1845 incorporation of the Republic of Texas into the United States of America, which was admitted to the Union as the 28th state on December 29, 1845.

List of battles fought in New Mexico: This list of battles fought in New Mexico is an incomplete list of military and other armed confrontations that have occurred within the boundaries of the modern U.S. State of New Mexico since European contact.  The region was part of the Viceroyalty of New Spain from 1535 to 1682, New France from 1682 to 1762, Kingdom of Spain from 1762 to 1800, French First Republic 1800 to 1803, and Mexico from 1803-1848.  Portions of New Mexico were under the administrative control of the Republic of Texas from 1836 to 1846.  Full administrative control of New Mexico was established on February 2, 1848 with the signing of the Treaty of Guadalupe Hidalgo which ended the Mexican–American War.

List of battles fought in Colorado: This list of battles fought in Colorado is an incomplete list of military and other armed confrontations that have occurred within the boundaries of the modern US State of Colorado since European contact.  The region was part of the Viceroyalty of New Spain from 1535 to 1682, New France from 1682 to 1762, Kingdom of Spain from 1762 to 1800, French First Republic 1800 to 1803, and part of the United States of America 1803–present (boundaries were disputed by Spain).  The southern portion of Colorado was considered by Spain as part of its northern territories.  Large portions of Colorado were subsequently under the administrative control of Mexico from 1800 to 1835, and the Republic of Texas from 1836 to 1846.  Full administrative control of Colorado was established on February 2, 1848 with the signing of the Treaty of Guadalupe Hidalgo which ended the Mexican–American War.

Supporting paragraphs: Mexican Cession Texas annexation
Answer: 1845