This is again a very incomplete story.
I assume the “treaty” you speak of is the one Santa Ana made after his capture. It was never ratified by the Mexican Congress and did not have any validity under international law as agreements made by prisoners under duress are not considered valid.
The land between the Nueces and the Rio Grande was disputed. Texas and the US claimed the Rio as the border, while Mexico claimed the Nueces.
When the new president, Polk, entered office he ordered the US Army to occupy the disputed territory, which the Mexicans considered theirs. When the Mexicans attacked to evict the invaders, he reported to Congress that American troops had been killed on American soil.
While there was no doubt that American troops had been killed, it required a good deal of spinning to assert with a straight face that it was “on American soil.” Lincoln’s most famous speech during his sole term in Congress was to demand that Polk prove that the spot where the troops died was American soil at the time.
Of course the Mexicans refused to negotiate over the border. They had never recognized the validity of Texan independence and still viewed it as a Mexican province in rebellion.
While Congress declared war, it was widely considered by
Whigs and northerners to be the result of a proslavery conspiracy seeking land to expand the institution.
The Mexicans behaved very stupidly in the runup to the war and there was plenty of fault on both sides, but it is just inaccurate to refer to the Mexican War as an American response to Mexican aggression.
IIRC, Lincoln lost his congressional re-election bid because of his opposition to the Mexican War.