most of them are in the red states
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I don’t think that is necessarily true.
And although Texas is well armed, I would suspect the US military and the Mexican military are better armed.
First of all, no ones going to kill potentially hundreds of thousands of Americans for choosing to secede from the United States.
This isn't 1861. Americans will never kill their own countrymen over such a purely political decision again. We also don't have the animating issue of slavery this time around. The political and social will doesn't exist to hold any state that wants to exit the union against their will. Especially through the use of brute force.
Secondly, I don't know where you're getting your information from, but there are only about 1.5 million active duty troops in the entire U.S. military. There are more well armed citizens than that in the state of Texas alone.
And if you think that tanks and advanced weapon systems would even the odds for them, look no further than Iraq to see how that would play out. It's been estimated that a force of no more than fifteen thousand 'insurgents' held our military at bay for nearly ten years in that place.
If the Iraqi fighters had had even half the numbers we possess, the U.S. military could never have taken them down.
The dynamics in such a hypothetical contest would also prove daunting to the government. They'd be asking their troops to apply military force against their own people. That would not end well - for them.
And as far as the country of Mexico sending their military against a U.S. secessionist state --- huh?? Please explain why they'd commit troops to fight our citizens, and what they would gain by doing so. They haven't committed their troops to any conflict that I can think of in living memory.