| Both Henry Clay and Martin Van Buren had said before the 1814 presidential election got under way that the annexation of Texas would involve the United States in a boundary dispute with Mexico that might bring on war. It did. When Texas was a state within Mexico its boundary was the Nueces River, not the Rio Grande. But when Texans under Sam Houston (dubbed The Raven) defeated the Mexican general, Santa Anna, at the San Jacinto River in April 1836 it declared itself an independent republic. This new Republic of Texas claimed that the Rio Grande River as its southwestern border. General Santa Anna had signed a "treaty" after his defeat at San Jacinto which accepted the Rio Grande as the boundary. The Mexican Congress had rejected this agreement as having no legal status because it was made under force by a person unauthorized to conclude a treaty.
Almost as soon as the United States annexed Texas, Mexico broke off diplomatic relations. On June 15th, 1845 General Zachary Taylor was ordered to defend the "territory of Texas" along the Rio Grande. Sizeable United States forces were in the disputed area by late summer. Several months before fighting broke out the United States tried to buy from Mexico the area that the United States were soon to take by force of arms. Mexico refused to sell. In April 1846 an American reconnoitering force of 63 men was captured. In the accompanying skirmish 11 Americans were killed. The war was on. President James K. Polk appeared before Congress on May 11th to ask for a declaration of war because Mexico had "shed blood upon American soil." War was declared on May 13th.
The states south of the Ohio River and west of the Alleghenies were enthusiastically in favor of this war. Among the original thirteen states of the Old South was content with the annexation of Texas, while the Middle Atlantic states and New England were hostile to the war. To the Northeast, the Mexican War was a plot to acquire more slave states. This was reflected in the make-up of our armed forces. While many of the officers were West Point men from all sections of the nation, the volunteers came principally from the West and South. James Russell Lowell penned some bittered doggerel which expressed well enough the sentiment of New England Boys.
"They may talk o' Freedom's airy
Tell they're pupple in the face;
It's a grand gret cemetary
Fer the barthrights of our race;
They just want this Californy
So's to lug new slave-states in
To abuse ye, an' to scorn ye,
An' to plunder ye like sin."
Fighting during this war was often severe, but there was never any doubt about the final outcome. John C. Fremont, the "Pathfinder," took a leading part in the conquest of California. He had led scientific expeditions in the Rockies, Nevada, California, and Oregon regions before the Mexican War broke out. General Stephan Kearny carried the war into New Mexico from Forth Leavenworth on the Missouri River to Santa Fe, then westward along the Gila River and on to San Diego and Los Angeles. General Zachary Taylor's victory at Buena Vista ended the war in northern Mexico. General Winfield Scott's campaign from Vera Cruz to Mexico City brought the war to a close. On September 14th, 1848 the Marines broke through the walls of the mountain capital, Mexico City, and took possession of the "Halls of Montezuma." The war had cost the United States about 1,700 killed in battle and 11,000 victims of disease. Mexican losses were far greater.
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