Changes

In 1819, Monroe completed the Adams-Onis Treaty to annex Florida to the United States, and also added property north of the southern border of current Oregon (which we now call the "Northwest").

In 1819, Monroe completed the Adams-Onis Treaty to annex Florida to the United States, and also added property north of the southern border of current Oregon (which we now call the "Northwest").

 

In 1820, the successful "Missouri Compromise" resolved for several decades disputes about whether slavery would be permitted in new territories, and how to admit new states.  This Compromise was between the North and the South, neither of which wanted to give up power by adding a state friendly to the other side.  The Senate was evenly balanced at this time, and adding just one new state on one side would tip the balance in its favor.  At the time there were 11 free states and 11 slave states (notice that, perhaps surprisingly, Delaware and Kentucky were slave states).   

In 1820, the successful "Missouri Compromise" resolved for several decades disputes about whether slavery would be permitted in new territories, and how to admit new states.  This Compromise was between the North and the South, neither of which wanted to give up power by adding a state friendly to the other side.  The Senate was evenly balanced at this time, and adding just one new state on one side would tip the balance in its favor.  At the time there were 11 free states and 11 slave states (notice that, perhaps surprisingly, Delaware and Kentucky were slave states).