Although frequently simplified by students of the conflict, the causes of the Mexican War of 1846-1848 were complex. Relations between the two countries had been strained almost from the moment Mexico won its independence from Spain in 1821. Although a republican form of government was established in 1824, Mexico proved to be a republic in name more than in fact. Wracked by frequent revolutions, the nation remained weak and unstable and was often dominated by dictators. As a result of the disorder, the United States, France, and Great Britain lodged claims against the government for damages inflicted upon their nationals and property. The American claims were submitted to a commission for arbitration, which settled on a figure of about $2 million. When the Mexican government defaulted, sentiment among Americans for collecting the claims by force increased, and some urged that war be declared. Mexico's grievance against the United States focused on the issue of Texas. Already angered by America's aid to the Texas Revolution, the Mexican government became further alarmed when the movement to annex Texas to the United States gained momentum. Mexico had never recognized Texas's independence and made plans to recapture the area. As Congress debated the issue, Mexico made it clear that the permanent loss of Texas would be sufficient cause for war. Events moved swiftly following the passage of an annexation resolution on March 3, 1845, the day before James K. Polk assumed the presidency. Fears for the safety of Texas and rumors that Mexico would transfer California to Great Britain in lieu of its debt payment, combined with a new sense of national identity and destiny, heightened American sensitivity to Mexico's threats and moved Americans closer to a war spirit. Mexico recalled its minister in Washington and broke off diplomatic relations. In response, U.S. troops commanded by Gen. Zachary Taylor entered Texas to protect the region until annexation was completed. Mexico countered by dispatching an army to the south bank of the Rio Grande. Hoping to avoid war with Mexico (conflict with Great Britain over the Oregon country loomed), President Polk sent an emissary, John Slidell, to the Mexican capital with instructions not only to negotiate a settlement of the claims and Texas issues but also to offer to buy New Mexico and California. Slidell arrived in early December amid a wave of anti-American feeling, and the government refused to receive him. The Mexican president, who it was said favored conciliation with the United States, was overthrown in a military coup. He was replaced by an officer who announced his intention to restore Texas to Mexico while he made overtures to European nations for the establishment of a monarchy in Mexico in return for aid against the United States. Following the admission of Texas to the Union in December 1845, Taylor's army was ordered to the Rio Grande, the traditional boundary of the American claim to Texas dating back to the early years of the century. The opposing Mexican force received orders to attack the Americans, and in late April, the commanding general informed Taylor that hostilities had begun. An American patrol was ambushed north of the Rio Grande, followed quickly by a movement of the Mexican force across the river. The two armies clashed in the Battles of Palo Alto and Resaca de la Palma in early May 1846. Although outnumbered, Taylor's army was victorious in both engagements. Slidell's rebuff by the Mexican government and news of the first American losses along the Rio Grande persuaded President Polk and his cabinet to ask that Congress recognize a state of war with Mexico. The war resolution passed on May 13, with only token opposition. The United States speedily mobilized its manpower and matériel. Congress authorized the enlistment of fifty thousand volunteers, assigning quotas to the states closest to the fighting. The government increased the size of the regular military forces, appropriated money for the production of equipment, and requisitioned ships to carry the troops to Mexico. There were three areas of military operation. Taylor's army penetrated northern Mexico, occupied the important city of Monterrey, and defeated a larger Mexican army commanded by General Santa Anna at the Battle of Buena Vista on February 22-23, 1847. In the meantime, an army under the command of Stephen W. Kearny followed the Santa Fe Trail to New Mexico, occupied Santa Fe, and moved westward to the Pacific where it joined naval units in the occupation of California. Impatient to end the war, Polk opened a third operation against Mexico City itself. Commanded by Winfield Scott, an army made up largely of volunteers landed at Veracruz in March 1847 and marched inland, defeating the opposing forces in hard-fought battles at Cerro Gordo and in the Valley of Mexico. The capital was occupied in mid-September 1847. The Treaty of Guadalupe Hidalgo, which ended the war, was signed early in February 1848. Mexico ceded New Mexico and California to the United States and, in recognition of the loss of Texas, agreed to the Rio Grande boundary. In return, the United States assumed the claims of its citizens against Mexico and paid Mexico an additional $15 million to help the country achieve long-needed fiscal stability. The Mexican War was costly for the United States. Its military forces suffered almost thirteen thousand deaths, although only seventeen hundred were battle-related, the rest resulting from disease that swept through the army camps. Nevertheless, the war was popular. It was the first war covered by large numbers of correspondents, as the nation's press competed for war news. Some members of the Whig party and the abolitionists opposed the war, the former because they felt it was unconstitutional, the latter believing erroneously that it was part of a slaveholders' conspiracy to extend slavery. For many Americans, the war was a romantic venture in a distant and exotic land. The campaigns were often compared with the Spanish conquest of Mexico in the sixteenth century, which had recently been popularized by the historian William Hickling Prescott. The reliance on volunteers gave the conflict a democratic cast, stimulating notions of an American mission to restore republican government to a people oppressed by military rulers. America's triumph seemed to confirm the superiority of democratic institutions, and literary figures like Walt Whitman and James Fenimore Cooper saw it as part of a worldwide mission to extend democratic ideals. Like most wars, however, this one left serious questions in its wake. The issue of whether slavery should be allowed in the lands taken from Mexico, first debated in 1846, set in motion a constitutional debate between the North and South that would dominate future political discourse, eventually dividing the Union itself.