If Lincoln was able to keep the north and south together with nothing but some muskets and cannons and a handful of barely effective proto machine guns, I'm pretty sure the Federal Government could easily retain Texas with 21st century forces and literally decades of 20th century backup from cold war stockpiles. Legally, nor practically, could Texas (or any state) leave the union without congressional approval - of which no one in any real power to make such a decision is even seriously talking about. While Texas is a large economy, and "thriving", it would also immediately lose a ton of businesses the minute it is no longer part of the US due to the legal complications. Any manufacturing that involves shipping to or from other states would immediately stop because shipping across a state border is way different than exporting goods across a national boundary. These might be temporary issues that could be resolved with treaties and legal agreements, but that could take years to be implemented, and longer if the US decides to embargo Texas out of spite. Another major factor here, is that a super-majority of Texans have no interest at all in their state leaving the United States of America, and would probably take issue with the idea of such an absurdly unpopular decision. *source: every poll conducted on the subject* (this is the real reason why Texas wont be seceding any time soon.)
