If texas attempted to make a trade agreement with mexico, in opposition to the u. s. constitution, which establishes tariffs and importing conditions for goods made in mexico, the u. s. constitutional objection to texas' actions would be based upon

The principle of Supremacy

The Supremacy Clause of the United States Constitution identifies the Constitution and the federal laws pursuant to it as the supreme law of the land. Thus states are subordinate to this law and the actions of Texas would be deemed unlawful