There were two main political faction, proto-parties. The Houstonians wanted annexation, but the Lamarians wanted independence. They dissagreed on a lot of other things. A civil war in Texas was not impossible, and that would've brought in the US. All they were looking for was an excuse. Manifest destiny was, as a policy, constructed after the Anglo-American War of 1812, but the grounds were based in old English claims of all of North America north of the Central Mexican Plateau[1].
BTW, the Sliders scenario is ludicrous. Texas didn't have the power to control the disputed territory between the Neuces and the Grande, in eastern New Mexico [2], and up on the high plains [3]. The Rio Grande valley could be seized during the civil war in Mexico, but will have to be defended when Juarez wins. Not Easy. The Comanche and Kiowa will have to dealt with and the said proto-parties couldn't agree on that[4].
1. I insist that the US-Mexican War of 1845 would've happened and been the Anglo-Spanish War of 1845 had the US and Mexico failed in their bids for independence.
2. I wonder if the Santa Feans even knew they were in disputed territory.
3. It was controlled by neither Texan nor Mexican, but by Comanche and Kiowa.
4. Houstonians favored friedship, trade, and coexistence; Lamarians favored war, conquest, and expulsion.