Final answer:
The main ideas supporting the purpose related to the Mexican-American War include the reintroduction of slavery into national political debates, the land disputes and racial tensions following the Treaty of Guadeloupe Hidalgo, and the increasing sectional divisions that were attempted to be addressed by the Compromise of 1850 but ultimately led to the Civil War.
Step-by-step explanation:
The Mexican-American War and the annexation of Mexican territories had profound consequences for the United States in the mid-19th century. Key issues arose regarding the extension of slavery into new territories, and tensions between the North and South grew as each region had different economic and political interests regarding the spread of slavery.
Figures such as John C. Calhoun and Abraham Lincoln were apprehensive about the war's outcomes because it meant reintroducing the contentious issue of slavery into national debates, having previously been suppressed by the gag rule. Furthermore, the Treaty of Guadalupe Hidalgo not only expanded U.S. territory but also significantly increased sectional controversies.
The war led to the Mexican Cession, which resulted in numerous problems, including land disputes and racial tensions. Many Mexicans and Mexican Americans lost their lands in Texas, California, and New Mexico due to the U.S government questioning the legality of existing land deeds. These events set a pattern of violence and racial antagonism that has echoes even in contemporary times.
In an effort to navigate these contentious issues, national political leaders attempted to alleviate sectional tensions with the Compromise of 1850. However, subsequent laws and court decisions, such as the Fugitive Slave Act, Kansas-Nebraska Act, and the Dred Scott decision only deepened the divide, making it increasingly difficult to find a peaceful solution. The cumulative effects of these disputes and events eventually contributed to the secession of several southern states and the onset of the Civil War after Lincoln's election.