Wow the ignorance here is amzing!
Firstly, Texas is the only state in the untion that actually has the right to secede (yes, it's spelled secede, not suceede). There wouldn't be any military action because they have the legal right to do so, so no military action would be either legal or required. All they would have to do is to file the required declaration.
Secondly, military action from Mexico would not only be unlikely but is downright laughable. They can't even keep basic law enforcement at the moment. No doubt they couldn't afford or carry out such a military effort. The militia in Texas is second to none, and they are both well armed and pretty damn well trained ( I know because I used to be a member).
Thirdly, and not finally, Texas has a pretty self-contained economy and could easily survive without the rest of the Union. A great deal of the banking, insurance, and agricultural industries are based in Texas. They can easly support themselves with little adjustment.
The issue of Texas secession comes up every few years and is always nothng more than talk. While they have the legal right they are very unlikely to follow through with it. Such talk is always amusing but serves to do nothing more than display people's lack of understanding of the law. In the end it really isn't likely to happen.
The talk of such is pretty amusing just the same.