[GOPcapitalist]

And the Mexican's repudiated the agreement that Santa Anna had made.

...putting them in breach of that agreement. After all, it wasn't Texas' fault that the Mexican government decided to overthrow their dictator a few months after Texas won its independence.

Of course Santa Anna returned to power in 1838, deciding he wasn't going to honor his agreement either making for a second breach of it. But by that time the Republic of Texas, as geographically defined under the treaty, was already recognized in the community of nations of the world so Santa Anna's whining didn't matter.

[fortheDeclaration]

The point is that the Mexicans certainly did not (rightly or wrongly) hold to the Rio Grande as being the boundry.

Thus, as far as they were concerned the border was still an open issue.