Final answer:
Slavery became a national issue in the U.S. as a result of territorial expansion, economic interests, and political debates, culminating in the Civil War after the election of Abraham Lincoln.
Step-by-step explanation:
The question of slavery emerged as a national issue in the United States through a series of historical developments and debates, fundamentally rooted in the country's westward expansion and economic differences between the North and the South. The Constitutional Convention set the stage with compromises that attempted to balance the interests of slaveholding states with those of non-slaveholding states. The Article VI of the 1787 Northwest Ordinance showed early intentions to restrict the spread of slavery, but it did not provide a lasting solution.
As the nation grew, new territories raised the question of whether they would enter the Union as slave or free states. The Missouri Compromise of 1820 and the Compromise of 1850 attempted to appease both sides, but only served as temporary measures. The invention of the cotton gin and the resulting boom in the cotton economy made slavery even more entrenched in the South. The acquisition of lands from the Louisiana Purchase and the Mexican-American War further complicated the issue.
Political events such as the Kansas-Nebraska Act, the Dred Scott decision, and the Fugitive Slave Act escalated tensions, highlighting the deepening divide over slavery. The formation of the Republican Party and the election of Abraham Lincoln, an opponent of slavery's expansion, made the issue paramount, leading to the secession of southern states and ultimately the Civil War. In summary, slavery became a contentious national issue as America expanded westward, pitting economic and political interests against each other, and leading to a national crisis that could not be resolved through compromise.