OK, maybe that might work for on-the-ground defense up to a point. But without sufficient counter attack, how do you deal with superior naval, air, and missile capabilities capable of scorching Texas? What about things like nuclear deterrents.
It’s sounds as if you are assuming that the military en masse would descend upon Texas. I feel quite certain that there would be insurrection among the troops and many would side with Texas (because it is their home state or because their sympathies lie with the cause of freedom) and much of the fighting would end up being “in house” between factions in the branches of the military.
You deal with the air power, with Stinger missles. It worked for Afghanistan. Call it "Operation Charlie Wilson".
You deal with naval power by retreating from the coast til they get tired of shelling you. If they land a ground force, every man has an AK47 and knows how to make IEDs, remember? It'll be Iraq to the 12th power. If they try a naval blockade, cut a deal with the Columbian traffickers (who are using Soviet subs to transport coke), to break the embargo.
As for missiles and nukes.... Short term, you publish the names, photos, and home addresses of every person in the decision chain, and let ALL your citizens know. If the button is pushed, the elites who pushed it will have to live in bullet proof bubbles for the rest of their lives, because, to quote Dune: "One day, one of us will get you!"
Long term, develop your own damn missles, nukes, and star-wars defense. All the frustrated scientist of the US will join you. "Houston, we do NOT have a problem!"