We shouldn't confuse Tex-Mex with Mexican or even Tejano (Texas Mexican) cuisine. When you think of Tex-Mex think of cheese enchiladas served with American or Cheddar cheese, chile con carne or even chili gravy; crispy tacos; and, as mentioned above, combination platters. This style of cooking came into style in the late 1940s-early 1950s in Texas, and has continued down to the present but with more additions from Tejanos (breakfast tacos, nopalitas, fajitas, barbacoa, tamales, and flour tortillas) and Mexicans (salsa mole, tacos, guacamole, ceviche, frijoles negros, tamales de Oaxaca).
The cuisine in Texas during and before the Civil War was plain old Tejano/Texas Mexican food, not Tex-Mex.
Spoken with love from a Texan who loves his state's cuisine with all his heart...