When Texas successfully secured its independence from Mexico in 1836, the Texans immediately applied for statehood. The U.S. Congress rejected them, so Texas established a republic and operated as an independent nation for nine years. Early in 1845, Congress relented and offered statehood. The only problem lay in the designation of Texas’s border with its former owner. Though the Mexican government had never recognized Texas’s independence, Mexico had not seriously tried to bring the recalcitrant state back into its union. Upon learning of the state’s annexation into the United States, the Mexican government was willing to let Texas go, but only on the condition that the borders follow the land grants Mexico had originally given to American settlers in the 1820s. Those borders stretched from the Nueces River in the south to the Red River in the north, territory that today encompasses central and east Texas. The Texans, however, claimed the Rio Grande as their border with Mexico, and claimed it to its source, which meant Mexico would have to cede about three times as much land, including its main northern settlement at Santa Fe. If the United States accepted the Texas claim to the Rio Grande, Mexico promised war.

American President James Polk sent John Slidell to Mexico to negotiate the purchase of the disputed territory and anything else Mexico might be willing to sell (such as California). The Mexicans not only refused his $15 million offer, they refused to recognize his very presence in their capital. This diplomatic insult, slight though it may have been, was fuel for the expansionist fires burning in American society, fires that Polk stoked in his election campaign. Coupling this incident with Mexico’s refusal to pay any damage claims for raids their army had conducted in Texas during the republic period, Polk felt justified in threatening Mexico.

Polk sent troops under General Zachary Taylor from New Orleans to Texas, ordering them to cross the Nueces and establish a presence along the north shore of the Rio Grande. Taylor began building Fort Polk and Fort Brown near the mouth of the river in March 1846. In mid-April, Mexican forces ambushed and captured a cavalry patrol. The Mexicans felt justified because they considered their country invaded as soon as American forces crossed the Nueces. For Polk, however, it was the final justification for war. He sent a message to Congress in early May, saying, “American blood has been shed on American soil” (a view not shared by Mexico). Congress agreed and declared war.

After two fairly easy victories in early May at Palo Alto and Resaca de la Palma, Taylor drove Mexican forces back across the Rio Grande. His forces crossed the river in June and worked their way upstream along the southern bank. In the meantime, the Mexican government had promoted Antonio Lopez de Santa Anna to command their forces. As dictator in 1836, Santa Anna had been defeated at San Jacinto, and it was he who signed the document that the Texans claimed gave them their independence. In 1844, Santa Anna had been removed from power a second time and exiled to Cuba. Some military historians regard him as one of the worst generals ever, but he had the ability to rise to leadership positions in Mexico over and over again.

Taylor arrived outside Monterrey with about 6,000 troops in the middle of September. He anticipated little difficulty in capturing the city despite the fact that the Mexicans had fortified the high ground around the city and dug extensive defenses across the more level approaches. The battle for Monterrey took three days, but the defending Mexican general asked for terms after American forces attacking from two directions had captured the high ground and were making their way through the city, which Taylor occupied, allowing the Mexican army to withdraw.

Meanwile, volunteer units were forming in the United States. The largest belonged to Stephen Kearny, a regular army colonel leading 1,500 frontiersmen, who marched westward from Kansas in the summer of 1846. He and his men were assigned to secure the New Mexico Territory, and by mid-August they raised the American flag over Santa Fe, declaring it and the territory to be U.S. possessions. Not a shot was fired on the campaign. In September, Kearny and 300 men marched for California. They arrived in December to find that forces from Oregon under John C. Fremont, along with naval forces under the command of John Sloat, and then Robert Stockton, had liberated the California Territory but were facing a popular uprising around Los Angeles. Stockton’s sailors and marines joined with Kearny’s small force to secure Los Angeles by early January. Mexican resistance in the territory ended; the only struggle yet to come was between Kearny and Fremont over who was actually in command in the territory.

As Kearny marched through the Southwest toward California, several hundred men from his original force left Santa Fe and headed south. Alexander Doniphan and his men enjoyed singular success in their expedition. They captured El Paso after a brief fight in late December; after a month of rest and recreation in the city, Doniphan’s force marched for the city of Chihuahua. Another brief battle (with two killed and seven wounded while inflicting 800 casualties on the Mexican force) gave them control of that town, followed by another month of rest and relaxation. They next marched for Monterrey to join with Taylor’s forces, arriving there too late for Taylor’s last major battle, at Buena Vista. They marched to the coast, sailed for New Orleans, were mustered out of service, and went home. They had claimed north-central Mexico for the United States by right of conquest, having accomplished the entire mission without regular army troops, orders, or leadership.

Northern Mexico was coming under American control, but Taylor was having his problems at Monterrey. Even though he was winning his battles, and had extended his hold southward to the town of Saltillo, the government was reining him in. President Polk, a Democrat, feared Taylor’s rising popularity, and he wanted to derail any future run Taylor might make toward high office as a Whig. The president ordered him to go on the defensive, but Taylor chafed at these orders. The latter widened his hold on the area around Saltillo, and ran into Mexican forces under Santa Anna. The Mexican force of 20,000 had marched across the desert to reach Buena Vista, south of Saltillo, in late February. After difficult fighting on 23 February 1847, Taylor’s forces held their ground, and Santa Anna retreated. It would be the last major battle in the north.

Newly arrived American General Winfield Scott siphoned off some of Taylor’s forces and sailed for Vera Cruz. Scott captured the port city fairly easily, and began to march west for Mexico City. Santa Anna had returned to the capital after his defeat at Buena Vista, and began to direct the defense of the city. Scott’s advance through difficult terrain was harassed periodically by Mexican guerrillas, but he approached the city by late August. The two sides negotiated a ceasefire to discuss peace terms, but Santa Anna was only buying time to improve his defenses. By early September, the armistice was over and Scott’s forces drew nearer to the city.

Unwilling to have Scott negotiate a peace treaty and make him even more popular than his military victories were doing, President Polk sent Nicholas Trist to Mexico to talk with the Mexican government. Congress had returned to Whig dominance after the last election, and the Whigs did not support the war. Polk hoped to secure the original goals of this war: the disputed area of Texas and possibly American possession of California. Certainly Mexico had suffered enough to concede to these demands.

Trist entered Mexico City under a flag of truce and found the government in chaos and unwilling to negotiate. He withdrew and sent word to Polk of his lack of success. The message took six weeks to reach Washington, owing to travel time, and the reply took equally as long. Trist’s original communication had been sent in late July, and the reply did not arrive in Mexico City until November: Forget the negotiations and come home. By then Scott had captured Mexico City, Santa Anna had been deposed, and the new Mexican government was negotiating with him. Still operating under his original orders, Trist was in a quandary. Should he continue to negotiate, or follow the latest directive to go home? He stayed.

In the meantime, Polk learned of the success in Mexico City and saw an opportunity to gain not only Texas and California, but also all of Mexico. He sent a new directive to Trist to forget the original instructions and demand complete capitulation. That message arrived after Trist had negotiated the treaty and left for Washington. Under the terms of the Treaty of Guadalupe- Hidalgo, Mexico ceded the disputed area of Texas and gave up all lands west toward the Pacific. In return, the United States would pay the originally offered $15 million, plus $3.25 million in damage claims held against Mexico by American citizens. The United States had just fought a year and a half to force Mexico to sell land.

When Trist arrived in Washington, unaware of the president’s last message, he proudly visited the White House to display the fruits of his labors. Polk was furious, almost murderous. The United States might have taken all of Mexico without paying anything, if only Trist had better understood his president’s expansionist attitudes. Had he exercised personal initiative and seized the moment, he could have seized the entire country. Polk did not want the treaty, but knew that congressional opposition would not allow him to continue the war, so he reluctantly signed it and sent it to the Senate for ratification. No one in the Senate liked it, either, thinking that it took too much, or too little, from Mexico; they ratified it as a compromise. The Mexican government were loath to part with any land at any price, but they were in no position to make demands; they ratified it as well.

The Treaty of Guadalupe-Hidalgo is one of the great might-have-beens of history. The future of the United States and all of Latin America would have been radically altered if the United States’ southern border had become the Yucatan Northern Mexico was coming under American control, but Taylor was having his problems at Monterrey. Even though he was winning his battles, and had extended his hold southward to the town of Saltillo, the government was reining him in. President Polk, a Democrat, feared Taylor’s rising popularity, and he wanted to derail any future run Taylor might make toward high office as a Whig. The president ordered him to go on the defensive, but Taylor chafed at these orders. The latter widened his hold on the area around Saltillo, and ran into Mexican forces under Santa Anna. The Mexican force of 20,000 had marched across the desert to reach Buena Vista, south of Saltillo, in late February. After difficult fighting on 23 February 1847, Taylor’s forces held their ground, and Santa Anna retreated. It would be the last major battle in the north.

Questions aside, there were concrete results from the war. The United States achieved its “manifest destiny” by reaching from sea to shining sea. Within a year of possessing California, gold was discovered and the rush was on. Having two distinct coastlines gave the United States the opportunity to expand overseas trade to the Orient as well as to Europe. The United States benefited greatly from the land gained, despite the fact that the slavery question over this new land almost directly led to civil war. Combat experience gained in Mexico showed itself in just a few years when junior officers under Taylor and Scott became senior officers in Union and Confederate uniforms. In terms of foreign relations, Latin America began to view the United States with increasing suspicion. The nation that had seemed a defender of the region with the Monroe Doctrine in the 1820s came to be viewed as a bully taking what it wanted from a weaker neighbor on trumped-up charges. The United States never lost that reputation, and did little in succeeding years to ameliorate it.