The correct answer to this open question is the following.
What happened to Mexicans at the end of the Mexican-American War (1846-1848) who were living in the area affected by the Treaty of Guadalupe-Hidalgo (1848) was the following.
Mexicans who lived in that territories suffered from the pain and destruction caused by the war. There was a considerable loss of property and much damage in towns. However, after the signing of the Guadalupe-Hidalgo Treaty, Mexicans who lived in those territories were allowed to have United States citizenship.
The treaty was signed in February 1848 and Mexico had to give the United States government the states of California, Utah, New Mexico, Arizona, and parts of Wyoming and Colorado.