So let's say that there is a dragged out War of 1812 that splits the north and south. Southerners moving west will be moving into Texas. There is thus more manpower in Texas and so the Texan Revolution is more successful. Not that it wouldn't be successful because even if Texas lost 1 revolution, there would be a second Texan Revolution which Texas would be more likely to win.
This attracts even more settlers to Texas. Something like the Mexican American War would still happen but because of Manifest Destiny, Texas would reach the California coast first. The British would at least want to integrate Seattle Bay into Canada. Something like this would happen as far as territory:
Texas would be on the opposite side of the USA in WWII. It would be on the side of the Axis powers and destroy the USA. The allies might still win WWII but Texas will have destroyed the USA. Then Texas does the arduous task of integrating the territory it defeated in WWII. Then it would look like this:
How big would the Texan military have to be for this to occur? Would Texas have to inherit the military in the South?
EDIT: Texas goes to war against the US during WWII. I don't think it would matter much what year of WWII Texas went to war as long as the Texan-American war ends before or when WWII ends which is in the year 1945.