5th January » The United States House of Representatives votes to stop sharing the Oregon Territory with the United Kingdom.
28th January » The Battle of Aliwal, India, is won by British troops commanded by Harry Smith (army) named Sir Harry Smith.
31st January » After the Milwaukee Bridge War, Juneautown and Kilbourntown unify as the City of Milwaukee called Milwaukee, Wisconsin.
4th February » The first Mormon pioneers make their exodus from Nauvoo, Illinois, westward towards Salt Lake Valley.
10th February » First Anglo-Sikh War: Battle of Sobraon British defeat Sikhs in final battle of the war
19th February » In Austin, Texas the newly formed Texas state government is officially installed. The Republic of Texas government officially transfers power to the State of Texas government following the annexation of Texas by the United States.
20th February » Polish insurgents lead an Kraków Uprising e.g uprising in Kraków to incite a fight for national independence.
14th April » The Donner Party timeline named Donner Party of Settler or pioneers departs Springfield, Illinois, for California, on what will become a year-long journey of hardship, cannibalism, and survival.
25th April » Thornton Affair: Open conflict begins over the disputed border of Texas, triggering the Mexican–American War.
1st May » The few remaining Mormons left in Nauvoo, Illinois, formally dedicate the Nauvoo Temple (Latter Day Saints) called Temple.
8th May » Mexican–American War: The Battle of Palo Alto Zachary Taylor defeats a Mexican force north of the Rio Grande in the first major battle of the war.
11th May » President James K. Polk asked for and received a Declaration of War against Mexico, starting the Mexican–American War
13th May » Mexican–American War: The United States declares war on Mexico.
23rd May » Mexican–American War: President Mariano Paredes of Mexico unofficially declares war on the United States.
14th June » Bear Flag Revolt begins United States known as Anglo settlers in Sonoma, California, start a rebellion against Mexico and proclaim the California Republic.
15th June » The Oregon Treaty establishes the 49th parallel north named 49th parallel as the border between the United States and Canada, from the Rocky Mountains to the Strait of Juan de Fuca.
16th June » The Papal conclave, 1846 known as Papal conclave of 1846 concludes. Pope Pius IX is elected Pope beginning the longest reign in the history of the papacy.
7th July » Mexican–American War: American troops occupy Monterey, California or Monterey and San Francisco known as Yerba Buena, thus beginning the U.S. acquisition of California.
10th August » The Smithsonian Institution is chartered by the United States Congress after James Smithson donates $500,000.
22nd August » The Second Federal Republic of Mexico is established.
10th September » Elias Howe is granted a patent for the sewing machine.
12th September » Elizabeth Barrett Browning called Elizabeth Barrett elopes with Robert Browning.
14th September » Jang Bahadur and his brothers Kot massacre or massacre about 40 members of the Nepalese palace court.
19th September » Two French shepherd children, Mélanie Calvat and Maximin Giraud, experience a Marian apparition on a mountaintop near La Salette, Isère named La Salette, France, now known as Our Lady of La Salette.
23rd September » Astronomers Urbain Le Verrier known as Urbain Jean Joseph Le Verrier, John Couch Adams and Johann Gottfried Galle collaborate on the discovery of Neptune.
24th September » Mexican–American War: General Zachary Taylor captures Monterrey.
25th September » U.S. forces led by Zachary Taylor capture the Mexican city of Monterrey.
10th October » Triton (moon) e.g Triton, the largest moon of the planet Neptune, is discovered by English astronomer William Lassell.
16th October » William T. G. Morton first demonstrated ether anesthesia at the Massachusetts General Hospital in the Ether Dome.
26th December » Trapped in snow in the Sierra Nevada (U.S.) called Sierra Nevadas and without food, members of the Donner Party resort to cannibalism.
28th December » Iowa is admitted as the 29th U.S. state.