How did the Missouri Compromise affect the spread of slavery?
It did expand slavery by one state, but it also kept the balance between slave states and free states.
The Missouri Compromise mainly accepted Missouri's application for statehood as a slave state, and admitted Maine as a free state.
During the events leading to the Compromise, pro-slavery and anti-slavery citizens caused tension between the North and South. When Missouri wished to join the Union, the North criticized the fact that Missouri allowed slaves.
To preserve the delicate balance between slave states and free states, the Union also adopted Maine as a free state. This way, the balance is kept equal, and both Southerners and Northerners were quiet, if not satisfied or happy, about the issue on slavery involving Missouri.