So what essential fact am I missing that indicates that the US either fired on Mexican troops, marched into Mexican (NOT Texan) territory, or declared war first?
The terms of that 1828 treaty were clearly overcome by events when Texas gained its independence and then applied for admission. That treaty never contemplated an independent nation in that area whose citizens wished to join the US.
Most people in Mexico believed the use of arms was the only option available to defend their rights and territorial integrity. Thus, on July 6, 1846, President Mariano Paredes enacted the Congressional decree that sustained such principles in the following terms:
Article 1. The government, in the natural defense of the nation, will repel the aggression initiated and sustained by the United States of America against the Republic of Mexico, having invaded and committed hostilities in a number of the departments making up Mexican territory.
Article 3. The government will communicate to friendly nations and to the entire republic the justifiable causes which obliged it to defend its rights, left with no other choice but to repel force with force, in response to the violent aggression committed by the United States.2
Ah, but the territory being referred had become part of the US. While that may not use the term "declaration of war", it states a clear intention to invade the United States. Close enough.
Oh, and what did Article 2 say?
You simply do not want to face the facts.
From that last link:
Shortly after Texas was admitted to the Union as the twenty-eighth state, President James K. Polk dispatched a special envoy, John Slidell, to Mexico City to settle the Texas boundary dispute and to arrange the purchase of California. The Mexican president, José Joaquín Herrera, had been willing to recognize an independent Texas but was under intense domestic pressure to reject United States annexation and Texas's expanded territorial claim. As a result, he refused to meet Slidell and began reinforcing Mexican army units along the Río Bravo del Norte. |
