Answer:
In short, the Mexican-American War was caused because of the United States' repeated encroachment on Mexican territory, such as its' annexation of Texas, which Mexico refused to recognize as being independent. Therefore, Mexico also refused to recognize the claimed border between the two nations.
Step-by-step explanation:
The immediate cause of the Mexican-American War was a disputed boundary between the United States and Texas on the Nueces Strip. Mexico did not recognize Texas as legitimate American territory and Texas admission to the United States antagonized Mexican officials and citizens.