The Annexation of Texas, the Mexican-American War, and the Treaty of Guadalupe-Hidalgo, 1845–1848. During his tenure, U.S. President James K. Polk oversaw the greatest territorial expansion of the United States to date.

In 1845, the Republic of Texas was annexed to the United States of America, becoming the 28th U.S. state. Border disputes between the new state and Mexico, which had never recognized Texas independence and still considered the area a renegade Mexican state, led to the Mexican–American War (1846–1848).

Description: A map of the territory acquired from Mexico including the annexation of Texas in 1845, California, Utah Territory, and New Mexico Territory in 1848, and the Gadsden Purchase in 1853.

The resulting treaty required Mexico to cede 55 percent of its territory including the present-day states of California, Nevada, Utah, most of Colorado, New Mexico and Arizona, and a small portion of Wyoming.

The Republic of Texas, acting in conformity with the wishes of the people and every department of its government, cedes to the United States all its territories, to be held by them in full property and sovereignty, and to be annexed to the said United States as one of their Territories, subject to the same constitutional provisions with their ot...

Nov 9, 2009 · While Mexico didn’t follow through with its threat to declare war, relations between the two nations remained tense over border disputes, and in July 1845, President Polk ordered troops into...

The government of the United States, by certain joint resolutions, bearing date the 1st day of March, in the year A.D. 1845, proposed to the Republic of Texas, then a free, sovereign and independent nation, the annexation of the latter to the former as one of …

We are required to “cede” to the United States, “all public edifices, fortifications, barracks, ports, harbors and navy and navy yards, docks, magazines, arms, armaments, and other property and means pertaining to the public defence belonging to the said Republic of …

In 1845, when Texas joined the United States, Mexico insisted the United States had a right only to the territory northeast of the Nueces River. The United States argued in turn that it should have title to all land between the Nueces and the Rio Grande as well.

The core of the treaty defined the "Mexican Cession," the territory that Mexico was obliged to cede to the United States as a result of the war. The Mexican War was the culmination of a series of conflicts between Mexico and the United States.