Samuel Morse sends "What hath God wrought" from Washington to Baltimore, inaugurating long-distance electric telegraphy.
War between the United States and Mexico following the US annexation of Texas; Mexico is defeated and loses about half of its territory.
At Massachusetts General Hospital, William Morton anaesthetises a surgical patient with ether in public, opening the era of surgical anaesthesia.
The discovery of gold at Sutter's Mill draws some 300,000 people to California from around the world, transforming the American West.
The treaty ends the Mexican–American War, with Mexico ceding California and much of today's Southwest to the United States.
The first women's rights convention in the United States adopts the Declaration of Sentiments, including a demand for women's suffrage.
Mexican troops under General Santa Anna storm the Alamo in San Antonio, killing its Texan defenders; the battle becomes a symbol of the Texas Revolution.