From 1846 to 1848, the United States of America and Mexico fought the Mexican-American War. There were many causes of the war, but the biggest reasons were Mexico's lingering resentment over the loss of Texas and the Americans' desire for Mexico's western lands, such as California and New Mexico. The Americans believed their nation should extend to the Pacific: this belief was called "Manifest Destiny."
The Americans invaded on three fronts. A relatively small expedition was sent to secure the desired western territories: it soon conquered California and the rest of the current US southwest. A second invasion came from the north through Texas. A third landed near Veracruz and fought its way inland. By late 1847, the Americans had captured Mexico City, which made the Mexicans agree to a peace treaty which ceded all of the lands the US had wanted.
But why did the US win? The armies sent to Mexico were relatively small, peaking at about 8,500 soldiers. The Americans were outnumbered in nearly every battle they fought. The entire war was fought on Mexican soil, which should have given the Mexicans an advantage. Yet not only did the Americans win the war, they also won every major engagement. Why did they win so decisively?
The US had Superior Firepower
Artillery (cannons and mortars) was an important part of warfare in 1846. The Mexicans had decent artillery, including the legendary St. Patrick's Battalion, but the Americans had the best in the world at the time. American cannon crews had roughly double the effective range of their Mexican counterparts and their deadly, accurate fire made the difference in several battles, most notably the Battle of Palo Alto. Also, the Americans first deployed the "flying artillery" in this war: relatively lightweight but deadly cannons and mortars that could be swiftly redeployed to different parts of the battlefield as needed. This advance in artillery strategy greatly helped the American war effort.
The American invasion from the north was led by General Zachary Taylor, who would later become President of the United States. Taylor was an excellent strategist: when faced with the imposingly fortified city of Monterrey, he saw its weakness right away: the fortified points of the city were too far from one another: his battle plan was to pick them off one by one. The second American army, attacking from the east, was led by General Winfield Scott, probably the best tactical General of his generation. He liked to attack where he was least expected and more than once surprised his opponents by coming at them from seemingly out of nowhere. His plans for battles such as Cerro Gordo and Chapultepec were masterful. The Mexican Generals, such as the legendarily inept Antonio Lopez de Santa Anna, were way outclassed.
Better Junior Officers
The Mexican-American War was the first in which officers trained at the West Point Military Academy saw serious action. Time and again, these men proved the value of their education and skill. More than one battle turned on the actions of a brave Captain or Major. Many of the men who were junior officers in this war would become Generals 15 years later in the Civil War, including Robert E. Lee, Ulysses S. Grant, P.G.T. Beauregard, George Pickett, James Longstreet, Stonewall Jackson, George McClellan, George Meade, Joseph Johnston and others. General Winfield Scott himself said that he would not have won the war without the men from West Point under his command.
Infighting Among the Mexicans
Mexican politics was extremely chaotic at that time. Politicians, Generals and other would-be leaders fought for power, making alliances and stabbing one another in the back. Mexico's leaders were unable to unite even in the face of a common enemy battling its way across Mexico. General Santa Anna and General Gabriel Victoria hated one another so badly that at the Battle of Contreras, Victoria purposely left a hole in Santa Anna's defenses, hoping the Americans would exploit it and make Santa Anna look bad: Santa Anna returned the favor by not coming to Victoria's aid when the Americans attacked his position. This is only one example of many of Mexican military leaders putting their own interests first during the war.
Poor Mexican Leadership
If Mexico's generals were bad, their politicians were worse. The Presidency of Mexico changed hands several times during the Mexican-American War. Some "administrations" lasted only days. Generals removed politicians from power and vice-versa. These men often differed ideologically from their predecessors and successors, making any kind of continuity impossible. In the face of such chaos, troops were rarely paid or given what they needed to win, such as ammunition. Regional leaders, such as governors, often refused to send any aid at all to the central government, in some cases because they had serious problems of their own at home. With no one firmly in command, the Mexican war effort was doomed to fail.
The American government committed plenty of cash to the war effort. The soldiers had good guns and uniforms, enough food, high-quality artillery and horses and just about everything else they needed. The Mexicans, on the other hand, were totally broke during the entire war. "Loans" were forced from the rich and the church, but still corruption was rampant and the soldiers were poorly equipped and trained. Ammunition was often in short supply: the Battle of Churubusco might have resulted in a Mexican victory had ammunition arrived to the defenders in time.
The war with the USA was certainly Mexico's biggest problem in 1847…but it wasn't the only one. In the face of the chaos in Mexico City, small rebellions were breaking out all over Mexico. The worst was in the Yucatán, where indigenous communities which had been repressed for centuries took up arms in the knowledge that the Mexican army was hundreds of miles away. Thousands were killed and by 1847 the major cities were under siege. The story was similar elsewhere as impoverished peasants rebelled against their oppressors. Mexico also had enormous debts and no money in the treasury to pay them. By early 1848 it was an easy decision to make peace with the Americans: it was the easiest of the problems to solve, and the Americans were also willing to give Mexico $15 million as part of the Treaty of Guadalupe Hidalgo.
Eisenhower, John S.D. So Far from God: the U.S. War with Mexico, 1846-1848. Norman: the University of Oklahoma Press, 1989
Henderson, Timothy J. A Glorious Defeat: Mexico and its War with the United States.New York: Hill and Wang, 2007.
Hogan, Michael. The Irish Soldiers of Mexico. Createspace, 2011.
Wheelan, Joseph. Invading Mexico: America's Continental Dream and the Mexican War, 1846-1848. New York: Carroll and Graf, 2007.