Texas declared its secession from the Union on February 1, 1861, and joined the Confederate States on March 2, 1861, after it had replaced its governor, Sam Houston, who had refused to take an oath of allegiance to the Confederacy. As with those of other states, the Declaration of Secession was not recognized by the US government at Washington, DC. Some Texan military units fought in the Civil War east of the Mississippi River, but Texas was more useful for supplying soldiers and horses for the Confederate Army. Texas' supply role lasted until mid-1863, when Union gunboats started to control the Mississippi River, which prevented large transfers of men, horses, or cattle. Some cotton was sold in Mexico, but most of the crop became useless because of the Union's naval blockade of Galveston, Houston, and other ports.

In the early winter of 1860, Texan counties sent delegates to a special convention to debate the merits of secession. The convention adopted an "Ordinance of Secession" by a vote of 166 to 8, which was ratified by a popular referendum on February 23.

From the Ordinance of Secession, which was considered a legal document, Texas also issued a declaration of causes spelling out the rationale for declaring secession. The document specifies several reasons for secession, including its solidarity with its "sister slave-holding States," the U.S. government's inability to prevent Indian attacks, slave-stealing raids, and other border-crossing acts of banditry. It accuses northern politicians and abolitionists of committing a variety of outrages upon Texans. The bulk of the document offers justifications for slavery saying that remaining a part of the United States would jeopardize the security of the two. The declaration includes this extract praising slavery, in which the Union itself is referred to as the "confederacy":

We hold as undeniable truths that the governments of the various States, and of the confederacy itself, were established exclusively by the white race, for themselves and their posterity; that the African race had no agency in their establishment; that they were rightfully held and regarded as an inferior and dependent race, and in that condition only could their existence in this country be rendered beneficial or tolerable.

— Texas Secession Convention, A Declaration of the Causes which Impel the State of Texas to Secede from the Federal Union (February 1861).

At this time, African Americans comprised around 30 percent of the state's population, and they were overwhelmingly enslaved. According to one Texan, keeping them enslaved was the primary goal of the state in joining the Confederacy:

Independence without slavery, would be valueless... The South without slavery would not be worth a mess of pottage.

— Caleb Cutwell, letter to the Galveston Tri-Weekly (February 22, 1865).