How did the slavery issue get tied up with manifest destiny in the 1840?

As a result of this theory that white settlers were 'destined' to colonize the whole of what today is the USA, new territories were settled and these territories were coming up for the status of US States. The discussion about slavery had already been going on for some time between the slave-holding South and the Northern States. In the until then existing States the South had an equality if not a majority position in the discussion, and the issue had for the time being been more or less settled in the Missouri Compromise.

The new States however threatened to upset the balance of power between the slave-holding States and the rest, and the Southern States therefore insisted that newly established States should also formally become slave-holding States. The Northern States opposed this, as did a sometimes large part of the inhabitants of the newly colonized areas.