What did the Missouri Compromise of 1820 accomplish?

The Missouri compromise had successfully stalled a potential early civil war overall.

The document itself agreed there would be no more new slave states above the 39th parallel, the last one being Missouri. Speaking of Missouri, the compromise had to be made due to the small states decision.

Now, it's important to understand the balance both slave and free states wanted to keep so neither could dominate each other. Missouri was a new state which wanted to be admitted slave. This caused the north to make the new free state of Maine as a balance to Missouri.