Final answer:
The most significant issue causing tension during the incorporation of new states was the expansion of slavery and whether new states would enter the Union as free or slave states. This controversy highlighted the economic and political rifts between the North and South and ultimately contributed to the Civil War.
Step-by-step explanation:
The biggest issue that caused tensions between politicians and states when territories wanted to become new states in the United States was the controversy over the expansion of slavery. The debate intensified with the question of whether new states, such as Missouri, Texas, and California, should enter the Union as slave or free states.
The acquisition of new territories from Mexico after the Treaty of Guadalupe Hidalgo in 1848 escalated the debate, contributing to national tensions. Economic and political implications of the peculiar institution polarized the North and South, with Southerners seeing the expansion as a means to preserve their way of life and Northerners viewing it as a threat.
Several legislative actions, such as the Missouri Compromise, the Compromise of 1850, and the Kansas-Nebraska Act, attempted to resolve or avoid the question of slavery in the territories. However, these measures only provided temporary relief.
National political leaders found the issue of territorial expansion further complicated by the discord over slavery. Eventually, this irreconcilable conflict over slavery and statehood led to the secession of several Southern states and the onset of the Civil War. The question of extending slavery into new territories was a critical factor that shaped the political landscape of the time.