Of course not.
OTOH, if Texas was unwilling to meaningfully negotiate an accounting of separation damages suffered by each side and allow for the peaceful withdraw of US assets and personnel, but instead began hostilities against same, then it would be otherwise.
One possible worst case: the remaining US government is politically unwilling to come to the defense of the new country of Texas in the event of armed incursion across its southern borders.
Having lived in Texas your statement is hilarious. The Mexican Army is going to invade Texas? LOL.