There were multiple reasons for the Mexican War. One immediate cause was the American annexation of Texas; the Mexican government regarded this a declaration of war, and removed the Mexican minister from Washington. Another cause was American claims against Mexico arising from the Mexican revolutions.
Following Mexico’s independence from Spain, American and European cartographers fixed the Texas border at the Neuces River. Prior to Texas’s independence, the Neuces River was recognized as the northern boundary of Mexico. Spain had fixed the Neuces as a border in 1816, and the United States ratified it in the 1819 treaty by which the United States had purchased Florida and renounced claims to Texas.
Throughout the 1820s, Americans settled in the vast territory of Texas, often with land grants from the Mexican government. Their numbers soon alarmed the authorities, however, who prohibited further immigration in 1830. In 1834 General Antonio Lopez de Santa Anna established a dictatorship in Mexico, and the following year Texans revolted when the new government abolished slavery.
Santa Anna defeated the American rebels at the celebrated siege of the Alamo in early 1836. On 21 April 1836 General Sam Houston with some 1,000 Texans under his command annihilated the 1,400-man army of Santa Anna. The Battle of San Jacinto lasted 18 minutes, and won Texas its independence from Mexico. When Texas declared its independence, it claimed as its territory an additional 150 miles of land, to the Rio Grande River. For almost a decade, Texas remained an independent republic.
At first, the American government strove to preserve peace with the goal to purchase New Mexico and California. The Jackson and Van Buren administrations feared both diplomatic trouble and the political consequences of admission of a new slave state; they therefore did not press the issue. The frontrunners in the 1844 presidential nominations, Democrat Martin Van Buren and Whig Henry Clay, announced they were against immediate annexation of Texas. In response, Southern democrats managed to block Van Buren’s nomination, allowing dark horse James K. Polk to come to the forefront. He campaigned for the acquisition of both Texas and Oregon. Clay, seeing the popularity of Polk’s stand, began hedging on the question of annexation, thus causing a defection of anti-slavery Whigs from the party, a defection which probably cost him the election.
After Texas gained its independence from Mexico, its voters overwhelmingly supported annexation into the United States. Although Mexico broke relations with the United States over the issue of Texas statehood, the most contentious issue was the new state’s border: Texas claimed the Rio Grande River; Mexico argued that the border stood far to the north along the Nueces River. Meanwhile, settlers were flooding into the Mexican territories of New Mexico and California at a time when many Americans claimed that the United States had a “manifest destiny” to expand westward to the Pacific Ocean.
On 01 March 1845 Congress employed its power to admit new states, and annexed Texas by a majority vote. On the fourth of March 1845 Polk was duly inaugurated. The annexation of Texas brought into the Union all or parts of Colorado, Kansas, New Mexico, Oklahoma, and Wyoming. The admission of Texas into the Union helped to speed the disintegration of the national political parties. Many Van Buren Democrats, convinced that southerners had trampled over their rights, left the party for the Free-Soil or Republican parties. Political ideologies and political parties were thus becoming sectional, making the Civil War almost unavoidable. The United States also adopted Texas’s position and claimed the Rio Grande as the border, helping to provoke war with Mexico.
The “Army of Observation” commanded bv General Zachary Taylor was deployed to Corpus Christi, at the mouth of the Nueces River, to protect newly annexed Texas in the summer of 1845. The force consisted of 5 regiments of infantry, 1 regiment of dragoons, and 16 companies of artillery.
Mexico broke diplomatic relations with the United States and refused to recognize either the Texas annexation or the Rio Grande border. President James Polk sent a special envoy, John L. Slidell, to propose cancellation of Mexico’s debt to United States citizens who had incurred damages during the Mexican Revolution, provided Mexico would formally recognize the Rio Grande boundary. Slidell was also authorized to offer the Mexican government up to $30 million for California and New Mexico. At that time, New Mexico embraced much of what is now the southwestern United States, not just the present state of New Mexico.
Between Slidell’s arrival on December 6, 1845, and his departure in March 1846, the regime of President Jose Herrara was overthrown and a fervently nationalistic government under General Mariano Paredes seized power. Neither leader would speak to Slidell. When Paredes publicly reaffirmed Mexico’s claim to all of Texas, Slidell left in a temper, convinced that Mexico should be “chastised.”
The agent for chastisement was already in place. In mid-January 1846, more than 3,500 troops commanded bv General Taylor moved south under President Polk’s order from Corpus Christi to a location on the north bank of the Rio Grande. Advancing on March 8 to Point Isabel, the US troops found that the settlement had been burned by fleeing Mexicans. By March 28, the troops were near the mouth of the Rio Grande across from the Mexican town of Matamoros. The Rio Grande formed part of the border between the United States and Mexico.
Polk claimed the move was a defensive measure, and expansionists and Democratic newspapers in the United States applauded his action. Whig newspapers said that the movement was an invasion of Mexico rather than a defense of Texas. General Taylor sent one of his officers across the river to meet with Mexican officials. The Mexicans protested the movement of the American troops to the Rio Grande. They said the area was Mexican territory. The movement of American troops there, they said, was an act of war. For almost a month, the Americans and the Mexicans kept their positions. While newspapers in Mexico called for war, General Pedro de Ampudia warned, “If you insist in remaining upon the soil of the department of Tamaulipas, it will clearly result that arms, and arms alone, must decide the question.”
General Ampudia’s prediction came true on 25 April 1846, when General Taylor received word that a large Mexican force had crossed the border a few kilometers up the river. A small force of American soldiers went to investigate. The Mexican cavalry attacked the mounted American patrol, killing five, wounding eleven, and capturing forty-seven. General Taylor quickly sent a message to President Polk in Washington. It said war had begun.
President Polk sent his war message to Congress on May 11 asserting, “Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon America’s soil.” He asked congress to give him everything he needed to win the war and bring peace to the area. On May 13, Congress declared war, with a vote of 40-2 in the Senate and 174-14 in the House. President Polk signed the war bill. Later, Polk wrote: “we had not gone to war for conquest. But it was clear that in making peace we would, if possible, get California and other parts of Mexico.”
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