What effect did mexicos winning its independence from spain have on american settlements in the southwest?

In on September 27th, 1821, Mexico has gotten its independence from Spain, At first it didn't effect with the American Settlements, But soon after it started to effect them because Mexico was poor and a harsh country to adapt to. So a lot of Mexicans moved to southwest settlements because life was easier than in Mexico, this issue is still occurring in this present time, Mexican people attempting to hop the border and have a better life, unfortunately as soon as they hop the border, they are considered illegal immigrants.