How was slavery a major issue as the nation wrestled with expansion?

As the United States expanded to the West, slavery was a major issue because it had to be decided whether the recently acquired land would become free states or slave states. There was a statu quo situation in which the Northern free states and the Southern slave states were balanced, there was the same number of one and the other. This was reflected in Congress as well, which permitted that slavery wasn't encouraged but it wasn't forbidden as well. With the new land and the new states, the policy was to keep things balanced, but of course there were anti-slavery and slavery groups in society that promoted otherwise.