How would Texas create valid deterrents and defend itself from foreign invasion including the U.S.?
Funny, you actually hit on the real problem if Texas was to become a separate country.. The invasion would be everyone trying to move there.. LOL
Insurgents always beat organizsed, mechanized militaries. So you don't need an army, you just need an entire male population, armed and ready to become insurgents. Since the Old Testament is a good tribal warfare manual, let's consult it --
From twenty years old and upward, all that are able to go forth to war in Israel: thou and Aaron shall number them by their armies Numbers 1:3
Every male, 20 years and up, of sound mind and body, gets an AK47 -- the most successful insurgent weapon ever -- and is trained to use it, and keeps it at home with a stash of ammo. If they wish to buy handguns, Barrett 50 cals, etcs at their own expense, as a supplement to the mandatory AK47, they are free to do so.
Start universal marksmanship at age 10. Teach guerrilla tactics, IEDs, molotov cocktails, etc in High School, instead of wasting time in sports. Sun Tzu is mandatory reading. At 20, enroll them as militiamen, according to the Bible.
For extra security.. Every police station and city hall has a stash of extra rifles and ammo, plus RPGs for vehicles and Stingers for aircraft. All men must be taught how to use these weapons also.
Nobody will attack, nobody will invade.
It’s Texas...Everyone and their dog knows how to make a rocket launcher out of a weed-eater and gunpowder.