The Texan people assemble in front of the Texan parliament to peacefully and patriotically make their voices heard. "Long live the Kaiser!", "Free Texas", their actions are perfectly legal under Texan law. The local police is present to ensure order is maintained. Additionally general strikes commence across the country to make it economically unviable to occupy the land. Many industrialist destroy their industrial machinery and if they don't their workers destroy it for them as part of the strike. The unjustified horrific bombings by Madrid have only severed to unite the Texan people around their national pride. The mass murder of government officials by are formal allies has only made martyr's of thousand of people now seen as the first heroes of the Texan Movement. There deaths inspired more people to join the protest as they were popular figures in Texas before there murder. The parliament declines to replace them so their can be no collaborations government to help the invaders. Regular rallies are to be held in every major city with demonstrators not obligated to disperse as their right to peaceful assembly, speech, and protest is all protected in the Texan government charter. Texan army units on the mainland stand down and dis-band themselves to avoid further tension. All the weapons held by the army simply vanish into the already very heavily armed Texan population.
Good luck occupying a massive nation with roughly the same population as yours.