Americans and immigrants had one great economic oppurtunity that stands out others, the opportunity to own land and to get large amounts of land for very cheap amounts of money.
The US had superior fire power. Better generals and junior officers. People in Mexico were also fighting among themselves with poor leadership. The United States also had better resources.
American Indians, or native Americans, had many different laws passed on them. They were both and independent state, yet apart of America. The Indian removal act was one of many passed on them. These different laws gave them certain rights such as the right to their own type of government, or forced them to live in certain areas.