Texas can lock down their neighbors and just wait for Cali. Cali can only come from one direction and there's no one geographically close to TX that's a threat.
Sit and wait for a Western invasion. Cut off Cali in west Texas and choke off the supply lines.
Why can Cali only come from one direction? They can send troops into the Mojave streaming across Arizona/NM and hit Texas from the west. Or they can send the troops down from San Diego into Mexico, come up from below and hit Texas from the South. Or they can send troops through Oregon/Nevada across the empty Midwest and hit Texas from the north. Or they can use their massive naval advantage to send ships all the way through the Panama Canal and hit Texas from the sea.
Texas doesn't have the same opportunity because California's borders are all protected by mountain ranges that would force the Texas military into narrow passes in order to reach the actual assets and Texas doesn't have the necessary naval advantage to win a sea battle. California could amass their forces near those passes and destroy the oncoming Texas army like sitting ducks.
Lockdown the Gulf oil so there's no fuel to mount air attacks for other states. Basically, every state has a plan but without fuel...they can only attack on the ground. Texas can use tanks plus air support. Shift the bulk of the forces to the Eastern 1/3 of TX.
Cali has the #3 best oil production in the USA and if push comes to shove they can always send the navy to Alaska and force them to put out.
Cali doesn't really need to attack because they have almost everything except for sufficient water. Luckily, Cali can take water with brute force from the neighbors.
If TX gets involved, it would be to lend resources to Cali neighbors to protect their water from Cali.
TX gotta make allies or we'd get crushed.
California supplies almost all its own water, it gets it from the NorCal mountains. And if we needed water from Oregon they'd oblige, we have far more money to bribe them than Texas plus Oregon ain't gonna help a distant state when they need our protection and could be invaded by Cali at any moment. Not to mention that all Cali would need to do to solve it's water problems is pull back on fruit/nut production in the central valley, they currently supply 2/3 of the nation's fruit and nuts, we could let most of that die cause we ain't feeding y'all anymore. (There's ONE cali nut producer that literally uses more water than every home in LA combined.)
I agree that Cali would be very secure and wouldn't need to attack anyone, in reality the California commies would just be self-sufficient and create a socialist utopia while all the rednecks in the midwest/southeast killed each other. Probably secure the borders at first and then slowly let in refugees from all the failed states in the rest of the county as you saw the error of your ways.
But I'm just throwing out advantages for the sake of argument.
Mid states produce a lot of agriculture but I’m still going with Florida. Rednecks, Duval brehs and Broward county brehs are crazy as all hell. They craycray for real. We might even get help from the Cubans .... nevermind they’re 
most of the time but def help from the islands right?
Annapolis is an okay-ish Naval port but we’d need to acquire Norfolk and Newport News to go beyond a brown water fleet and into blue water territory, especially with them 3-5 aircraft carriers based down there. We got Andrew’s Air Base but nothing wrong with obtaining more bases from VA. Maryland (excuse me, Murrland) would definitely become a major threat with that many Army and Air bases after bringing Virginia and Delaware into the fold in addition to a few Navy bases. DC would initially wanna remain independent but they would definitely get the “either get down or lay down” message when they see the Greater Murrland Prosperity Alliance Holy Army pull up around the city at the crack of dawn 
