If Texas went out—other states would join them. This would happen only if the President disregarded the US Constitution. Then Texas and others could form a new nation—The Constitutional States of America-—using the old USA Constitution around the Government. This would mean Civil War—if Texas seized nuclear arms and threaten to destroy New York or Washington-—It may end as soon as it starts. In any case—the USA would cease to exist.
If that happens, I would hope that our military honors their oath to defend the Constitution. We have almost come to that point.