Mexican troops clashed with Americans in the disputed area north of the Rio Grande claimed by both countries. Abraham Lincoln was in Congress at the time and demanded that the President prove that the spot where American blood was shed was US territory. So he was known as “Spotty” Lincoln.
Referring to "area north of the Rio Grande" as "disputed" was just a bunch of wordplay from Mexico that's been repeated over and over again in our lifetimes to support the liberal narrative of big bad Americans. Texas' succession from the new nation of Mexico was just as real as Mexico succeeding from Spain. After that, Mexico had no say in whether or not Texas joined the U.S. Mexico attacking above the Rio was a pure attack on the U.S.