Final answer:
The Mexican-American War began due to territorial conflicts and concluded with the Treaty of Guadalupe Hidalgo, which significantly expanded U.S. territory but also exacerbated domestic tensions over slavery.
Step-by-step explanation:
The Mexican-American War started over territorial disputes and ended with the Treaty of Guadalupe Hidalgo. This treaty had the U.S. paying $15 million and assuming $3.25 million in American claims against Mexico, while acquiring over 500,000 square miles of territory. Issues brought about by the war, such as the expansion of slavery, intensified sectional crises in the U.S. and set the stage for future domestic conflicts.
The acquisition of new territories exacerbated the debate over whether these lands would permit or prohibit slavery, fueling the sectional crisis that would eventually lead to the American Civil War. The war's aftermath shaped the nation's trajectory, showcasing the deeply rooted divisions over the institution of slavery that would define American politics for decades to come.