When the admission of new states threatened the balance of free and slave states in the union. What compromise was created?

The compromise created was: Missuri Compromise.

Explanation:

Missuri Compromise was created by the attempt of the United States Senate and House of Representatives in 1820, to try to balance the power between north and south. As part of the compromise, slavery was prohibit north while in south is was still happening.

This Compromise wanted to deal with slavery in new territories. It allowed slavery only in Missouri. This marked more the division between norh and south, and later was one other factor that lead to civil war.