Answer: Jefferson meant that the issue of slavery had not been resolved and would come to destroy the Union.
Explanation:
The Missouri compromise had brought the debate over slavery into the forefront, showing the ever increasing ideological differences between the North and the South. Although the compromise served to settle the matter with the admission of Maine as a free state and Missouri as a slave one, the ideological differences had been brought to light, as such it is considered as the beginnning of legislative and political movements that would culminate into the American Civil War.
