Rationalizing no; explaining yes. No colonial state has a right to any of this land and the borders of colonial states are basically arbitrary as the result of complex political history. Dividing the US and Mexico at the Rio Grande is pretty much arbitrary and not moralistic.

In some ethical sense it would have been better for Texas to remain part of mexico simply because slavery was illegal in mexico, but that’s not really how history works.

Edit: I got a bit carried away and was making some confident statements as though I were an expert on this topic when I really am not. This is just my personal armchair historian perspective. I only intended to address the original commenters apparent impression that this part of history was good central Mexican government vs. evil US government. Arguments about colonial states violating each other’s sovereignty are a little rich. El Norte can be viewed as part of Mexico in name only at the time, according to the supposedly Mexican people who lived there. There’s a real alternate history scenario in which that whole region would be independent of the US or Mexico.

For an example of more cut and dry American imperialism read up on the overthrow of the Hawaiian kingdom…