Opening hostilities

The Mexican government had established the border between Mexico and Texas at the Nueces River. Texas, however, set the border at the Rio Grande, giving Texas more land. A dispute over the territory between the rivers would lead to the Mexican-American War.

President James K. Polk had sent General Zachary Taylor and 1,500 American troops to the border along the Nueces River. Taylor arrived in July 1845 with his forces, but then was ordered by President Polk to cross into disputed territory. Taylor, with his army, marched to Corpus Christi, and later to north of the Rio Grande. A month later, the Mexicans attacked. Taylor's forces were too much for the Mexicans and drove them back beyond the Rio Grande River. President Polk took advantage of the skirmish and asked Congress for a declaration of war against Mexico.

[edit] Declaration of war

By then, Polk had received word of the Thornton Affair and added this to the rejection of Slidell as the casus belli. A message to Congress on May 11, 1846, stated that Mexico had "invaded our territory and shed American blood upon the American soil", even though it happened on Mexican soil (which was between Nueces River and Rio Grande, legally still Mexican). A joint session of Congress overwhelmingly approved the declaration of war, mostly because Democrats strongly supported the war. 67 Whigs voted against it on a key amendment, but on the final passage only 14 Whigs voted no, including Representative Abraham Lincoln. The United States declared war on Mexico on May 13, 1846, and Mexico officially declared war on July 7 (sometimes the manifest from President Paredes on May 23 is construed as the declaration of war, but only the Mexican congress had that power).

[edit] Opposition to the war

Whigs in both the North and the South generally opposed the war, while most Democrats supported it. Whig Congressman Abraham Lincoln contested the causes for the war, and demanded to know the exact spot on which Thornton had been attacked and U.S. blood had been shed. He was quoted as saying "Show me the spot." Whig leader Robert Toombs of Georgia declared:

"This war is a nondescript…. We charge the President with usurping the war-making power… with seizing a country… which had been for centuries, and was then in the possession of the Mexicans…. Let us put a check upon this lust of dominion. We had territory enough, Heaven knew." [Beveridge 1:417]

Northern abolitionists attacked the war as an attempt by slave-owners — frequently referred to as "the Slave Power" — to expand the grip of slavery and thus assure their continued influence in the federal government. Acting on his convictions, Henry David Thoreau was jailed for his refusal to pay taxes to support the war, and penned his famous essay, Civil Disobedience.

Former President John Quincy Adams also expressed his belief that the war was fundamentally an effort to expand slavery. In response to such concerns, Democratic Congressman David Wilmot introduced the Wilmot Proviso, which aimed to prohibit slavery in any new territory acquired from Mexico. Wilmot's proposal did not pass Congress, but it spurred further hostility between the factions.

[edit] Conduct of the war

After the declaration of war, U.S. forces invaded Mexican territory on two main fronts. The U.S. war department sent a cavalry force under Stephen W. Kearny to invade western Mexico from Fort Leavenworth, reinforced by a Pacific fleet under John D. Sloat. This was done primarily because of concerns that Britain might also attempt to occupy the area. Two more forces, one under John E. Wool and the other under Taylor, were ordered to occupy Mexico as far south as the city of Monterrey.

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Reasons To Differ