Why did conflicts occur between different groups living in the american southwest?

After a war between Mexico and America,  influx of Americans came into struggle with long term residents on land, resources, water and minerals. They also came into dilemmas such as legal, economic, and social discrimination. So they found it hard to protect their rights. US and Mexican laws clashed and many Mexicans,Mexican Americans and American Indians went to trial, but had to pay for attorneys', entrepreneurs' fees so went bankrupt. US also took little for Mexican legal concepts.