The results of the Mexican War (1846–48) brought Texas into serious conflict with the national government over the state's claim to a large portion of New Mexico. The claim was based on efforts by the Republic of Texas, beginning in 1836, to expand far beyond the traditional boundaries of Spanish and Mexican Texas to encompass all of the land extending the entire length of the Rio Grande. Efforts to occupy the New Mexican portion of this territory during the years of the republic came to naught (see TEXAN SANTA FE EXPEDITION).
In the early months of the Mexican War, however, federal troops, commanded by Gen. Stephen W. Kearny, easily occupied New Mexico. Kearny quickly established a temporary civil government. When Texas governor J. Pinckney Henderson complained to United States secretary of state James Buchanan, the latter replied that, though the matter would have to be settled by Congress, Kearny's action should not prejudice the Texas claim. By the provisions of the Treaty of Guadalupe Hidalgo, Mexico relinquished all claim to territory north and east of the Rio Grande. The treaty did not, however, speak to the issue of the Texas claim to that portion of New Mexico lying east of the river.
By this time New Mexico and all other lands ceded to the United States by Mexico had become embroiled in the slavery controversy. Southern leaders insisted that all of the new territory be opened to slaveholders and their human property. Northern freesoilers and abolitionists were determined to prevent such an opening and so resisted the claims of Texas to part of the area in question. Texas attempted to further its claim by organizing Santa Fe County in 1848, with boundaries including most of New Mexico east of the Rio Grande. In New Mexico military and civilian leaders then petitioned the federal government to organize their area into a federal territory. Texas governor George T. Wood responded by asking the legislature to give him the power and means to assert the claim of Texas to New Mexico "with the whole power and resources of the State." Soon afterward his successor, Peter H. Bell, made a more moderate request, asking only for authority to send a military force sufficient to maintain the state's authority in that area. Bell then sent Robert S. Neighbors west to organize four counties in the disputed area. Although he was successful in the El Paso area, Neighbors was not welcomed in New Mexico.
Publication of the report of Neighbors's mission in June of 1850 led to a public outcry in Texas. Some persons advocated the use of military force; others urged secession. Bell reacted by calling a special session of the legislature to deal with the issue. Before the session began, the crisis deepened. New Mexicans ratified a constitution for a proposed state specifying boundaries that included the territory claimed by Texas. Also, President Millard Fillmore reinforced the army contingent stationed in New Mexico and asserted publicly that should Texas militiamen enter the disputed area he would order federal troops to resist them. Southern political leaders responded by sending Governor Bell offers of moral and even military support.
Meanwhile, the United States Congress was grappling with the issue. On January 16, 1850, Senator Thomas Hart Benton of Missouri introduced a bill that would have had Texas cede all land west of 102° longitude and north of the Red River to the United States for $15 million. The bill would also divide Texas into two states. Soon afterward, Senator John Bell of Tennessee offered a resolution that would have divided Texas into three states. Then a Senate committee, chaired by Henry Clay of Kentucky, reported a bill that would have given Texas an unspecified sum in exchange for ceding all lands northwest of a straight line from the El Paso area to that point on the 100th meridian that intersects the Red River. None of these efforts proved successful.
Finally, Senator James A. Pierce of Maryland introduced a bill that offered Texas $10 million in exchange for ceding to the national government all land north and west of a boundary beginning at the 100th meridian where it intersects the parallel of 36°30', then running west along that parallel to the 103d meridian, south to the 32d parallel, and from that point west to the Rio Grande. The bill had the support of the Texas delegation and of moderate leaders in both the North and South. Holders of bonds representing the debt of the Republic of Texas lobbied hard for the bill, for it specified that part of the financial settlement be used to pay those obligations. The measure passed both houses of Congress in the late summer of 1850 and was signed by President Fillmore.
Though there was some opposition in Texas to accepting the proffered settlement, voters at a special election approved it by a margin of three to one. The legislature then approved an act of acceptance, which Governor Bell signed on November 25, 1850. The boundary act and four additional bills passed at about the same time, all dealing with controversial sectional issues, came to be known collectively as the Compromise of 1850.