I mean, lets look at the last civil war: the time between Lincolns election and the battle of fort Sumpter was about 6 months. The first serious battle was July, 9 months later.
Like, from the point the Texas government votes to leave, how long will a military response take? If they aren't immediately arrested by Capital police, and generally survive the police response, and the guard mostly splits, how long will it take just to organize the military response? As I've pointed to our other military campaigns, Iraq prep is a multi month operation. If not more.
And don't forget police actions can take multiple months. The wako siege lasted about 2 months. Bundy was a month long thing. I could see some siege of an FBI office, or the FBI sieging the mayoral office of some break away town, over a couple of months. Most people will want to see if the police action can solve the issues before committing to force on force military actions. That's months.
And of course waffling. If Texas votes to leave, I could see the West Virginia government waffle immensly. Yes, they're very red, but also right next to DC and some big military bases in Virginia. I can them trying to be formally neutral until a clear winner happens. Or at least Texas lasts for 6 or so months and its clear this is not going to end in some negotiated settlement, or see that the Texas resistance doesn't immediately collapse once a single tank rolls through.
Which of course is another reason to waffle on military comittment: once you have a texas guard armor engage federal armor in pitched battle, that's going to signal to everyone that this is a real thing and people need to commit to a side. And if that tank battle doesn't resolve in a decisive Federal Victory, that pushes the 20-40% of the people who might be pro Texas but quiet or waffling to commit openly now that it seems victory is possible. And the Texans now that if they commit to the battle and lose, they're all going to jail. So neither side will be eager to commit to a big battle.
And, well, if that battle shows this is a war rather than a quick suppression, its going to be months to build up the armies to fight it. Texas by itself with fairly minimal mobilization (1%) can raise 200,000 troops. That's about 1/5th the size of the US army, true, but as We've seen in our recent combat, for a decisive campaign you really want 3-1 or 10-1 numerical superiority.
And well, the US Army is not going to be 1 million after Texas has withdrawn. Halfish would have deserted more or less by that point, and you then have, well, places like West Virginia. If Texas is breaking away West Virginia supporting them if left to their own devices, so they may be worth a premtive invasion. Occupation rules of thumb suggests that needs about 20,000 troops there. And West Virginia is going to be one of the smaller places securing the Blue position requires occupation. So, Available forces to invade Texas would probably be closer to 100,000, which means invading at a 2-1 disadvantage.
So, invasion requires a build up. Suggesting we may then have something like a 6-10 month phony war as both sides gear up, neither side feeling ready for major offenses. Which also means we'll have close to a year where, as much as possible, America is going to try and increase production and exports: Building up the Texas army from about 20,000 guardsmen + army turncoats to the 200k-1 million man army they're going to need requires hundreds of thousands more missiles, thousands of tanks, and just huge quantities of material.
Whatever can't be manufactured domestically has to be purchased. Thus, Texas will have every incentive to maximize its oil and food exports to buy weapondry from whoevers willing to sell them. I could see in this period exports and imports to china increasing. And of course everyone is going to be stockpiling for when things hit the fan at some inevitable point.
So, it perfectly possible that we don't really see any large scale campaigns and fighting until some 2 years after the first succession.
Everything takes time.