March 1 – U.S. Congress passes a "Joint Resolution for Annexing Texas to the United States."
July 4 – The Texas Constitutional Convention votes to accept the United States annexation proposal; it drafts an Annexation Ordinance and State Constitution to submit to the voters of Texas.
Oct. 13 – Texas voters overwhelmingly approve annexation, the new state constitution and the annexation ordinance.
Dec. 29 – The U.S. Congress approves, and President James K. Polk signs, the "Joint Resolution for the Admission of the State of Texas into the Union." Texas becomes the 28th state.
Feb. 19 – Although Texas officially became a state on Dec. 29, 1845, the formal transfer of government does not take place until this date.
May 8 – The Battle of Palo Alto near Brownsville is the first major battle of the two-year Mexican War.
Feb. 2 – The Treaty of Guadalupe Hidalgo is signed, ending the War with Mexico and specifying the location of the international boundary.
Feb. 11 – The first railroad to actually begin operation in Texas is chartered by the state government. The Buffalo Bayou, Brazos and Colorado begins operation in 1853.
Nov. 25 – Texas' governor signs the Compromise of 1850, in which Texas gives up its claim to land that includes more than half of what is now New Mexico, about a third of Colorado, a corner of Kansas, the Oklahoma Panhandle, and a small portion of Wyoming in exchange for the United States' assumption of $10 million in debt; Texas keeps its public lands.
Two reservations are established for Indians in Northwest Texas: one for Comanches on the Clear Fork of the Brazos in Throckmorton County, the other for more sedentary Indian groups, such as Tawakonis, Wacos and Tonkawas, near Fort Belknap in Young County.
March 27 – Col. Robert E. Lee arrives in San Antonio. He serves at Camp Cooper on the Comanche reservation beginning April 9. He returns to Washington for a short time, coming back to San Antonio and Fort Mason in February 1860.
April 29 – Fifty-three camels arrive at the port of Indianola for a U.S. Army experiment using them for pack animals in the arid areas of the Southwest. They are quartered at Camp Verde, near present-day Kerrville, and are later joined by 41 more. The experiment lasted about 10 years.
Sept. 15 – The southern route of the Butterfield Overland Mail crosses Texas on its way between St. Louis, Mo., and the West Coast. Service is discontinued in March 1861 with the outbreak of the Civil War and is never resumed.
13 – Violent clashes between Juan "Cheno" Cortina and Anglo lawmen begin in the Brownsville area in the Lower Rio Grande Valley. Texas Rangers and federal troops eventually halt the so-called "Cortina War" in 1875.
July – Indians on the Northwest Texas reservations are moved by the federal government to reservations in Indian Territory (now Oklahoma).