Texashad been an independent republic for nine years, having gained independence in 1835 after defeating the Mexican Army at the Battle of San Jacinto. It was recognized as a nation by France, Great Britain, and the United States, but the goal of many Texans was to become part of the United States.
1835年，在聖哈辛托戰役中擊敗墨西哥軍隊後，德克薩斯州獲得獨立，至1845年已有9年的歷史了。雖然德克薩斯被法國、英國和美國承認是一個獨立的國家，但大部分德克薩斯人卻想歸屬美國。
Many Americans also felt that as part of the nation's "Manifest Destiny" to stretch from the Atlantic to the Pacific, the United States should take steps to annex Texas, the Oregon Territory, California, and the rest of the southwest. In 1844, James K. Polk was elected president, and he began to realize this dream. Just before President John Tyler left office in 1845, Congress passed a joint resolution to annex Texas. Polk then sent an agent to California to support a revolution against Mexican control if one should break out. He also sent John Slidell to Mexico with an offer to buy New Mexico and California for $30 million, but Mexican officials refused to talk to him. In 1846, the boundary dispute with Great Britain over the Oregon Territory was finally settled. The northern border of the United States would be extended along the 49th parallel to the Pacific Ocean.
很多美國人認為，美國應該採取措施吞併德克薩斯、俄勒岡區域、加利福尼亞和西南地區的其他領地，認為這是美國從大西洋向太平洋擴張的“天定命運”的一部分。1844年詹姆斯·K.波爾克當選總統，於是開始努力去實現這一夢想。在1845年上一任總統約翰·泰勒離職以前，美國國會就通過了吞併德克薩斯的議案。波爾克上任後，他先派代理人去加利福尼亞，支援當地人舉行的.反對墨西哥領導的重大變革，繼而又派約翰·斯萊德爾去墨西哥，與墨西哥政府談判以30,000,000美元低價贖買墨西哥和加利福尼亞，然而墨西哥官員拒絕與其交涉。1846年，就俄勒岡區域界限的爭議與英國達成一致。美國北部邊界線擴伸至沿太平洋北緯49°。
Trouble with Mexico erupted over a border dispute in Texas. The United States claimed that the border between Texas and Mexico should be the Rio Grande River, while the Mexicans claimed the border was farther north at the Nueces River. President Polk ordered General Zachary Taylor to take an army into the disputed region, and in April 1846, one of Taylor's patrols was attacked by Mexican troops. Claiming that "American blood has been shed on American soil" Polk asked Congress to declare war. Congress passed the declaration of war, although congressmen such as Abraham Lincoln and John C. Calhoun believed the war was just an excuse to seize land.
就德克薩斯邊界線的爭議致使美國與墨西哥兩國之間的動亂終於爆發。美國聲稱德克薩斯與墨西哥應該以格蘭德河為界，而墨西哥人認為邊界線應該設在往北一些的紐埃西斯河。波爾克總統命令扎卡里·泰勒將軍率兵前往爭議地區。1846年4月，泰勒的一支巡邏隊遭到墨西哥軍隊的襲擊。波爾克總統聲稱“美國人的血已經流到了美國的土地上”，請求國會向墨西哥宣戰。雖然亞伯拉罕·林肯和約翰·C.卡爾霍恩等議會會員認為宣戰只是侵佔領土的一個藉口，但議會還是通過了宣戰請求。
The Americans attacked on several fronts. Taylor's army advanced into the region south of the Rio Grande, winning victories at Palo Alto and Resaca de la Palma in May 1846. He captured Monterrey in September, and his victory over Santa Anna at Buena Vista in February 1847 ended fighting in northern Mexico. An army led by General Stephen Kearny captured Santa Fe and moved across the deserts and mountains into California. General Winfield Scott was sent to capture Mexico City. After victories at Vera Cruz and other cities along the way, Scott captured the capital city on September 14, 1847.
美軍在幾個地區開戰。泰勒的軍隊挺進格蘭德河以南地區，1846年5月，分別在帕洛阿爾託戰役和雷薩卡的拉帕爾瓦戰役中取得了勝利。同年9月他佔領了蒙特雷，1847年2月在布埃納維斯戰役中擊敗聖塔·安那後結束了墨西哥北部的戰鬥。另一支由斯蒂芬·科爾尼將軍領導的軍隊佔領了聖達菲，繼而又穿過沙漠和高山挺進加利福尼亞。溫菲爾德·斯科特將軍被派去攻佔墨西哥城。在攻佔維拉克魯斯和沿途的城市後，斯科特於1847年9月14日成功地佔領了墨西哥城這個首都城市。
Congress approved the Treaty of Guadalupe Hidalgo to end the war on February 2, 1848. Mexico agreed to give up all claims to Texas, establish the Texas border at the Rio Grande, and cede California and the New Mexico territory to the United States for $15 million. This added over 500,000 square miles to the United States.
1848年2月2日，美國議會通過了《瓜達盧佩·伊達爾戈條約》，結束了戰爭。墨西哥放棄了所有對德克薩斯的所有權，把格蘭德河訂為德克薩斯州的邊界，並同意以15,000,000美元低價把加利福尼亞和新墨西哥割讓與美國。這讓美國一下子就增加了5,000,000平方米的領土。
