Driven by the expansionist doctrine of "Manifest Destiny" under President James K. Polk, the U.S. annexed the Republic of Texas in 1845 and provoked war along the disputed Nueces-Rio Grande borderlands in April 1846. General Winfield Scott captured Mexico City in September 1847, forcing the signing of the Treaty of Guadalupe Hidalgo on February 2, 1848.
The treaty recognized the Rio Grande as the Texas border and ceded Alta California and Santa Fe de Nuevo México. In exchange, the United States paid Mexico $15,000,000 (roughly $550 million today) and assumed $3.25 million in claims of U.S. citizens against Mexico.
Just nine days before the treaty was signed in Mexico City, James W. Marshall discovered gold at Sutter's Mill on January 24, 1848. Neither Polk nor Mexican President Santa Anna knew of the gold reserves. Over 300,000 prospectors ('49ers) surged into California, turning San Francisco from a village of 200 into a global Pacific metropolis within two years.
Authentic Primary Source Document:
"The boundary line between the two Republics shall commence in the Gulph of Mexico, three leagues from land, opposite the mouth of the Rio Grande... thence along the middle of that river." — Treaty of Guadalupe Hidalgo, Article V (Feb 2, 1848)
Treaty of Guadalupe Hidalgo (Transcript Excerpt)
RATIFIED MAY 30, 1848
ARTICLE VIII: "Mexicans now established in territories previously belonging to Mexico... may elect to preserve the character of citizens of the Mexican Republic, or acquire the character of citizens of the United States... In the said territories, property of every kind, now belonging to Mexicans not established there, shall be inviolably respected."
ARTICLE IX: "The Mexicans who, in the territories aforesaid, shall not preserve the character of citizens of the Mexican Republic... shall be incorporated into the Union of the United States and be admitted at the proper time... to the enjoyment of all the rights of citizens."
