Why was the Treaty of Guadalupe Hidalgo important?

The Treaty of Guadalupe-Hidalgo, signed in 1848, ended the Mexican-American War. The US had declared war on Mexico nearly two years earlier, in 1846, over a dispute over Texas.

Texas, which had been a part of Mexico, won its independence in 1836 after a group of American colonists and settlers living in Texas rebelled against the Mexican government. The Americans then formed the Republic of Texas and began pushing for annexation by the US. In 1844 President John Tyler pressed Congress to pass a resolution annexing Texas. The Mexican government disputed the annexation and claimed that parts of Texas really belonged to Mexico, leading to border disputes between the US and Mexico over the boundaries of Texas. When President Polk ordered troops into these disputed lands, fighting broke out and the US officially declared war on Mexico.

The Mexican government surrendered after US troops captured Mexico City in 1847. The Treaty of Guadalupe-Hidalgo officially ended the war in 1848, adding 525,000 square miles of territory to the US. This territory included parts of present day Arizona, California, Colorado, New Mexico, Nevada, Wyoming, and Utah. The US paid Mexico $15 million for this vast expanse of land. In addition to greatly increasing the territorial size of the US, the Treaty of Guadalupe-Hidalgo and the Mexican-American War ignited old debates in the US, mainly over slavery. New territories meant new places where slavery could expand, rekindling debates about slavery that eventually led to sectional divisions that ignited the Civil War in 1861.