Final answer:
The issue of slavery in the US territories was politically heated in the 1800s because it threatened economic interests and the political balance between the North and the South, leading to legislative conflicts and increasing sectionalism that resulted in the Civil War.
Step-by-step explanation:
The Compromise of 1850, which included measures such as the Fugitive Slave Act and the admission of California as a free state, was an attempt to address these tensions. However, legislation and events like the Kansas-Nebraska Act and the Dred Scott decision only heightened sectional conflict, leading to greater polarization and ultimately, the Civil War.
The Mexican-American War and the resulting Treaty of Guadalupe Hidalgo brought new territories that had the potential to disrupt the delicate balance between free and slave states. Both the North and the South had interests in whether these new territories would permit slavery. Debates like these dominated political dialogues, culminating in Abraham Lincoln's election and the secession of several southern states.
The disputes over slavery in the territories symbolized the broader contest between two different social and economic systems and the desire of each region to maintain a political advantage. While southerners sought to expand slavery to preserve their agrarian way of life, northerners opposed its spread, fearing it would undermine the value of free labor and tip the scales of political power in favor of the South.