How were territorial expansion and the issue of slavery related in the united states?

Every time we added land the issue about if and when the area became a state was important. There was a balance in Washington and Congress between slave holding states and non-slave holding states (free states). The lawmakers from the free states were trying to eliminate slavery but couldn't manage to, so they instead tried to contain it. The Missouri Compromise contained it to any area south of the 36  30 parallel. Later they will attempt to keep it out of any lands won from the Mexican War with the Wilmot Proviso. These will all be over turned later with the Kansas-Nebraska Act that will state that the people living in the area should decide the matter themselves. This meant all open territory could be slave holding states. This would greatly increase their power and influence on politics in Washington.