What was the cause of the Mexican-American War?

Not sure if it’s within 12 months, but you could talk about the spreading of alt-right hate speech and whether that is covered by the right of freedom of speech? or, cover how during the midterm elections in some states many native american populations were declined to vote because they didn’t have the proper id, despite being citizens? also, the death of that journalist last month, who spoke freely and reported about the saudi government and how corrupted it is and was killed because of that (lack of free press/speech). just some ideas for this prompt, i hope at least one of them : )