There was yet an abundance of territory both north and south to be taken into the Union and, without any special plan or agitation, States were admitted in pairs, one free and the other slave.

In the meantime there was distinctly developed the idea of the possible or probable permanence of slavery in the South and of a rivalry or even a future conflict between the two sections.
When in 1819 Missouri applied for admission to the Union with a state constitution permitting slavery, there was a prolonged debate over the whole question, not only in Congress but throughout the entire country.
North and South were distinctly pitted against each other with rival systems of labor.

The following year Congress passed a law providing for the admission of Missouri, but, to restore the balance, Maine was separated from Massachusetts and was admitted to the Union as a State.
It was further enacted that slavery should be forever prohibited from all territory of the United States north of the parallel 36 degrees 30', that is, north of the southern boundary of Missouri.

It is this part of the act which is known as the Missouri Compromise.

It was accepted as a permanent limitation of the institution of slavery.