Did you know on March 2, 1836 Texas declared itself independent from Mexico? 175 years ago today, Texas was a nation separate from the United States as well. It would not become an official part of the U.S. until December 29, 1845, over 9 years after Texas declared its independence and named itself the Republic of Texas. It would become the 28th state to join the Union. Texans celebrate today as Texas Flag Day, a state holiday established in 1915 to observe Texas Independence Day.