 The dispute centered on whether the new states would be "free" states like the existing Northern states, in which slavery would be illegal, or whether they would be "slave" states like the existing Southern states, in which slavery would be legal.The Southern states wanted the new states to be slave states in order to enhance their own political and economic power, but the Northern states opposed this for their own political and economic reasons, as well as their moral concerns over allowing the institution of slavery to expand.In 1820, the U.S. Congress passed an agreement known as the "Missouri Compromise" that was intended to resolve the dispute.