The first effect of the Mexican-American War was the territorial gains made by the United States.  At the end of the war, Mexico, through the signing of the Treaty of Guadalupe Hidalgo, ceded California (including Alta California) and New Mexico (which including what is now Arizona, Utah, and Nevada) to the United States, and established the Rio Grande as Texas’ southern border once and for all.  This secession of land was completed for a final cost of approximately 15 million US dollars (and the U.S. replaced 3 million dollars in claims made against Mexico).  This land exchange, which had originally led to greater U.S. expansion, ultimately led to further conflicts, both international and national when on the terms of the U.S. situation (Goldfield 362).

      First, there was the internal dispute of slavery in the U.S.  Even before the Treaty of Guadalupe Hidalgo was signed, there had been arguments over what would become of the territories if they were to be annexed into the United States.  This was first seen in the Wilmot Proviso.  Created by David Wilmot, it stated, “as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico…neither slavery nor involuntary servitude shall ever exist in any part of said territory” (Goldstein 425).  This argument, given from a northern Democrat, typifies the viewpoint and desires of the North as a whole.  Wanting to maintain their power and the northern/southern balance (balance between antislavery and slavery), they wanted to outlaw slavery in the new territories as to keep from losing any political power/influence. This provision led Congress to division of Congress (Goldfield 425). 

Then, after further arguments over slavery ensued over the slave status of New Mexico and California, there were several attempts at compromising.  The one that succeeded, however, was the Compromise of 1850, which was originally thought of by Henry Clay.  This compromise consisted of four parts.  First, California would be a free state.  Second, the other western territories would vote over slavery.  Third, slave trade was stopped in Washington D.C. Finally, the Fugitive Slave Act was enacted.  Although, this was a compromise, it ignited the tensions between the Northerners and Southerners, who would constantly be at each other’s necks until the eruption of the Civil War, and thereafter (Goldfield 428). 

Next, there was the ill effect the war had on Mexico.  After losing almost half of its territory, Mexico was feeling all but kindness toward its neighbor to the north.  Angered by U.S. imperialism and territorial ideology, they no longer believed in anything the U.S. did or the information which they released.  The Mexican American War also led to various internal problems in Mexico.  There were various uprisings and governments and rulers were overthrown one after another.  Also, to make Mexico’s matters worse, after the war ended the United States continually interfered with Mexico’s government and society in almost every way possible (“Mexican War” 1).

The last effect of the Mexican-American War involved the former Mexican citizens who lived in the territories ceded by the Treaty of Guadalupe Hidalgo to the U.S.  These people were promised U.S. citizenship and their former lands by the United States.  When citizens of the eastern U.S. traveled into the new territories (another effect), the Mexican peoples’ claims on the lands were ignored as the “easterners” stole those lands and settled down.  Using the Treaty of Guadalupe Hidalgo as one of their key arguments, many of these Mexicans tried to regain their lost lands through lawsuits.  They, however, were unsuccessful (Griswold del Castillo, Richard 1).                 

     In conclusion there were various effects due to the Mexican-American War.  The U.S., on one side, both benefited and was hurt by the outcome of the war.  On one end, it gained massive amounts of territory, which was the equivalent of 66% of the U.S. before the reception of the territory.  On the other end, the aftermath of the war led to the disruptions in Congress and the build up of hatred between the North and South for each had a specific stance when it came to the problem/practice of slavery.  Mexico, on the other side, was for the worst by the war in all mannerisms.  Not only did it lose a vast amount of territory, but it also lost much of its stability due to the uprisings that took place during and after the war and had to deal with U.S. interference.