February 1 – Anson Jones, President of the Republic of Texas, signs the charter officially creating Baylor University. Baylor is the oldest university in the State of Texas operating under its original name.
July–August – In the United States Magazine and Democratic Review editor John L. O'Sullivan declares that foreign powers are trying to prevent American annexation of Texas in order to impede "the fulfillment [sic] of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions".[1] It is the first use of the phrase "Manifest Destiny".
October 21 – The New York Herald becomes the first newspaper to mention the game of baseball.
October 22 – The New York Morning News becomes the first newspaper to include a box-score of a baseball game.
December 2 – Manifest Destiny: U.S. President James K. Polk announces to Congress that the Monroe Doctrine should be strictly enforced and that the United States should aggressively expand into the West.
American journalist John L. O'Sullivan claims in a newspaper article that the United States has a "Manifest Destiny" to expand its borders. It's the second time he uses the term and it will have a huge influence on the American imperialistic movement of the 19th century.
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Media related to 1845 in the United States at Wikimedia Commons