The following outline is provided as an overview of and topical guide to Texas: Texas – second most populous and the second most extensive of the 50 states of the United States of America. Texas borders Mexico and the Gulf of Mexico in the South Central United States. Texas is placed in the Southern United States by the United States Census Bureau. The Republic of Texas joined the United States as the 28th state on December 29, 1845. Texas joined the Confederate States of America during the American Civil War from 1861 to 1865 but was readmitted to the Union in 1870.