What unresolved conflict was made worse by adding more territory to the united states in the early to mid-1800's

The debate over slavery. Adding more territory raised the question of whether slaves would be allowed in the West (and, subsequently, whether the West would align with the slaveholding or the free states in the conflict). Controlling the West meant either proliferating or stopping the expansion of slavery, depending on who got it. New Western states meant more power to either the slave states or the free ones in the Senate.