It was a very troublesome thing. At first, as the United States wanted to expand, they kind of forced the Mexican government to leave some of the colonies and they kept pushing westward. There were many wars but after the Mexican independence from Spain was won and after the borders settled south of California, things were pretty quiet and the relations calmed down and became political and economical.
Its always been a rough relationship here is a way to get exact answers go to this wikipedia link https://en.wikipedia.org/wiki/Mexico%E2%80%93United_States_relations
