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January 4 – The United States House of Representatives votes to stop sharing the Oregon Territory with the United Kingdom.
January 30 – The City of Milwaukee is incorporated in Wisconsin, merging Juneautown, Kilbourntown and Walker's Point following the Milwaukee Bridge War of 1845. Solomon Juneau is elected first mayor.
February 11 – Many Mormons begin their migration west from Nauvoo, Illinois to the Great Salt Lake, led by Brigham Young.
February 18 – In Austin, Texas, the newly formed Texasstate government is officially installed.
February 27 – The Liberty Bell is cracked while being rung for George Washington's birthday.
April 26 – Mexican–American War: Open conflict begins over border disputes of Texas' boundaries.
May 8 – Mexican–American War – Battle of Palo Alto: Zachary Taylor defeats a Mexican force north of the Rio Grande at Palo Alto, Texas in the first major battle of the war.
June 14 – Mexican–American War: The California Republic declares independence from Mexico.
June 15 – Bear Flag Revolt: American settlers in Sonoma, California start a rebellion against Mexico and proclaim the California Republic.
June 16 – The Oregon Treaty establishes the 49th parallel as the border between the United States and Canada, from the Rocky Mountains to the Strait of Juan de Fuca.
June 20 – The first officially recorded baseball game using modern rules developed by Alexander Cartwright is played in Hoboken, New Jersey.
July 8 – Battle of Monterey: Acting on instructions from Washington, D.C., CommodoreJohn Drake Sloat orders his troops to occupy Monterey and Yerba Buena thus beginning the United States annexation of California.
August 11 – Establishment of the Smithsonian Institution in Washington, D.C.
September 21–24 – Battle of Monterrey: General Pedro de Ampudia and the Mexican Army of the North are defeated by General Zachary Taylor's Army of Occupation, comprising U. S. Regulars, U. S. Volunteers and Texas Rangers. The battle ends after three days of fighting with 120 U. S. soldiers killed, 360 wounded, and 43 missing. The Mexicans suffer 367 killed and wounded.
November 31 – The Donner Party becomes snowbound.
December 29 – Iowa is admitted as the 29th U.S. state (seeHistory of Iowa).
Undated[]
The portion of the District of Columbia that was ceded by Virginia in 1789 is re-ceded to it.
John Denny, buffalo soldier and Medal of Honor recipient (died 1901)
Pugsey Hurley, British-born burglar, river pirate and underworld figure in New York City
Peter Remondino, Italian-born physician, author, first president of the San Diego Board of Health, and co-founder of San Diego's first private hospital (died 1926)
Mary Foot Seymour, businesswoman and writer (died 1893)
Thomas Shaw, buffalo soldier and Medal of Honor recipient (died 1895)
DeVoto, Bernard Augustine (1960). The year of decision, 1846. With an introductory essay by Catherine Drinker Bowen. Boston: Houghton Mifflin. OCLC3963759.