February 19 - 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 Texas' annexation by the United States.
March 27 - Mexican-American War: Siege of Fort Texas.
April 14 - The Donner Party of pioneers departs Springfield, Illinois, for California, on what will become a year-long journey of hardship, cannibalism, and survival.
April 25 - Thornton Affair: Open conflict begins over the disputed border of Texas, triggering the Mexican-American War.
May 1 - The few remaining Mormons left in Nauvoo, Illinois, formally dedicated the Nauvoo Temple.
May 8 - 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.
May 13 - Mexican-American War: The United States declares war on Mexico.
May 23 - Mexican-American War: President Mariano Paredes of Mexico unofficially declares war on the United States.
May 24 - Mexican-American War: General Zachary Taylor captures Monterrey.