Explain how the Treaty of Guadalupe Hidalgo changed the United States' southern border.

It doubled the size of the United States.

Explanation:

The Treaty of Guadalupe Hidalgo had changed the United States' southern border because Mexico agreed to give up all claims to Texas and accept the Rio Grande as the boundary of that United state of America. The United States and Mexico signed the treaty of Guadalupe-Hidalgo on February 2, 1848. Due to this treaty, the size of Mexico is halved whereas doubled the size of the United States.