The heated struggle in Congress over the admission of Missouri into the
Union first brought prominently forward the agitation of the slavery
question. This struggle, which lasted from 1818 to 1821, threatened the
very existence of the Union. Jefferson wrote from Monticello:--
"The Missouri question is the most portentous one that has ever
threatened the Union. In the gloomiest moments of the Revolutionary War
I never had any apprehension equal to that I feel from this source."