On May 13, 1846, the U.S. Congress voted in favor of President James Polk’s request to declare war on Mexico in a dispute over Texas. Under the threat of war, the United States had refrained from annexing Texas after the latter won independence from Mexico in 1836.

But in 1844, President John Tyler restarted negotiations with the Republic of Texas, culminating with a treaty of annexation.

The treaty was defeated by a wide margin in the U.S. Senate because it would upset the slave state-free state balance between North and South and risked war with Mexico, which had broken off relations with the United States.

But shortly before leaving office and with the support of President-elect Polk, Tyler managed to get a congressional resolution passed and then, on March 1, 1845, signed into law. Texas was admitted to the union on December 29 of that year.

While Mexico didn’t follow through with its threat to declare war, relations between the two nations remained tense over border disputes, and in July 1845, President Polk ordered troops into disputed lands that lay between the Nueces and Rio Grande rivers.

In November, Polk sent the diplomat John Slidell to Mexico to seek boundary adjustments in return for the U.S. government’s settlement of the claims of American citizens against Mexico and to make an offer to purchase California and New Mexico.

After the mission failed, the U.S. Army under General Zachary Taylor advanced to the mouth of the Rio Grande, the river that the state of Texas claimed as its southern boundary.

Mexico, claiming that the boundary was the Nueces River to the north of the Rio Grande, considered the advance of Taylor’s army an act of aggression and in April 1846 sent troops across the Rio Grande. Polk, in turn, declared the Mexican advance to be an invasion of U.S. soil, and on May 11, 1846, asked Congress to declare war on Mexico, which it did two days later, starting the Mexican-American War.

WATCH: The Struggle for Mexican Independence

Following the defeat of the Mexican army and the fall of Mexico City in September 1847, the Mexican government surrendered and peace negotiations began. The war officially ended with the February 2, 1848, signing in Mexico of the Treaty of Guadalupe Hidalgo.

The treaty added an additional 525,000 square miles to United States territory, including the land that makes up all or parts of present-day Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming.

Mexico also gave up all claims to Texas and recognized the Rio Grande as America’s southern boundary. In return, the United States paid Mexico $15 million and agreed to settle all claims of U.S. citizens against Mexico.

Although Polk’s war was successful, he lost public support after nearly two bloody and costly years of fighting. Additionally, the controversial war reignited the slavery extension debate that would ultimately result in the Civil War in the 1860s.

Polk did not seek re-election after his first term, and died at age 53 in June 1849, three months after leaving office. Zachary Taylor, who became a national hero during the Mexican-American War, ran for president in the 1848 election and won. However, 16 months after his inauguration, Taylor became ill and died.

Treaty of Guadalupe Hidalgo (1848). National Archives. 
The Treaty of Guadalupe Hidalgo. Library of Congress: Hispanic Reading Room.
On this day, the Treaty of Guadalupe Hidalgo is signed. National Constitution Center.