It was a controversial war that bitterly divided American public opinion. And it was the war that gave young officers named Ulysses S. Grant, Robert E. Lee, Thomas ("Stonewall") Jackson, William Tecumseh Sherman, and George McClellan their first experience in a major conflict.

The underlying cause of the Mexican War was the movement of American pioneers into lands claimed by Mexico. The immediate reason for the conflict was the annexation of Texas in 1845. After the defeat at San Jacinto in 1836, Mexico made two abortive attempts in 1842 to reconquer Texas. Even after these defeats, Mexico refused to recognize Texan independence and warned the United States that the annexation of Texas would be tantamount to a declaration of war.

In early 1845, when Congress voted to annex Texas, Mexico expelled the American ambassador and cut diplomatic relations. But it did not declare war.

President Polk told his commanders to prepare for the possibility of war. He ordered American naval vessels to position themselves outside Mexican ports. And he dispatched American forces in the Southwest to Corpus Christi, Texas.

Peaceful settlement of the two countries' differences still seemed possible. In the fall of 1845, the President offered $5 million if Mexico agreed to recognize the Rio Grande River as the southwestern boundary of Texas. Earlier, the Spanish government had defined the Texas boundary as the Nueces River, 130 miles north and east of the Rio Grande. No Americans lived between the Nueces and the Rio Grande, although many Hispanics lived in the region.

The United States also offered up to $5 million for the province of New Mexico--which included Nevada and Utah and parts of four other states--and up to $25 million for California. Polk was anxious to acquire California because in mid-October 1845, he had been led to believe that Mexico had agreed to cede California to Britain as payment for debts. Polk also dispatched a young Marine Corps lieutenant, Archibald H. Gillespie, to California, apparently to foment revolt against Mexican authority.

The Mexican government, already incensed over the annexation of Texas, refused to accept an American envoy. The failure of the negotiations led Polk to order Brigadier General Zachary Taylor to march 3,000 troops southwest from Corpus Christi, Texas, to "defend the Rio Grande" River. Late in March of 1846, Taylor and his men set up camp along the Rio Grande, directly across from the Mexican city of Matamoros, on a stretch of land claimed by both Mexico and the United States.

On April 25, 1846, a Mexican cavalry force crossed the Rio Grande and clashed with a small American squadron, forcing the Americans to surrender after the loss of several lives. On May 11, after he received word of the border clash, Polk asked Congress to acknowledge that a state of war already existed "by the act of Mexico herself...notwithstanding all our efforts to avoid it." "Mexico," the President announced, "has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil." Congress responded with a declaration of war.

The Mexican War was extremely controversial. Its supporters blamed Mexico for the hostilities because it had severed relations with the United States, threatened war, refused to receive an American emissary or to pay the damage claims of American citizens. In addition, Mexico had "invaded our territory and shed American blood on American soil." Opponents denounced the war as an immoral land grab by an expansionistic power against a weak neighbor that had been independent barely two decades.

The war's critics claimed that Polk deliberately provoked Mexico into war by ordering American troops into disputed territory. A Delaware Senator declared that ordering Taylor to the Rio Grande was "as much an act of aggression on our part as is a man's pointing a pistol at another's breast." Critics also argued that the war was an expansionist power play dictated by an aggressive Southern slave owners intent on acquiring more slave states.

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