What unresolved conflict was made worse by adding more territory to the US in the early to mid 1800s?

The unresolved conflict that was exacerbated by the expansion of U.S. territory in the early to mid-1800s was the issue of slavery. As new territories were acquired, particularly through the annexation of Texas and the outcomes of the Mexican-American War, the debate intensified over whether these areas would be designated as free or slave states. This tension ultimately contributed to the sectional divides between the North and South, leading to increased hostility and setting the stage for the Civil War.