Final answer:
Mexico was significantly larger at the time of its independence, encompassing what would become large portions of the southwestern United States. The Treaty of Guadalupe Hidalgo in 1848, following the Mexican-American War, resulted in Mexico ceding roughly half of its territory to the United States, shaping the southern border we recognize today.
Step-by-step explanation:
When Mexico gained independence from Spain in 1821, the new nation was much larger than the Mexico we know today. It included the territories that would become the modern U.S. states of California, Nevada, Utah, Arizona, New Mexico, Texas, and parts of Colorado, Wyoming, Kansas, and Oklahoma. During its early years of independence, Mexico, through the Plan de Iguala, was declared a constitutional monarchy, a nation with promises of social equality and protections for the Catholic Church. These extensive territories represented a vast area sparsely populated mainly by indigenous peoples and some Mexican citizens, largely unchanged before the influx of American settlers.
Over time, especially after the annexation of Texas by the United States in 1845 and the subsequent Mexican-American War, Mexico's territory was drastically reduced. The Treaty of Guadalupe Hidalgo in 1848 resulted in Mexico losing about half of its territory to the United States. This included all of the land north of the Rio Grande, thereby defining a significant portion of the present-day southern border of the United States. Through this treaty, Mexico also sought to protect the rights and properties of Mexican citizens who found themselves within the new borders of the United States.