This map is part of a series of 20 animated maps showing the history of The United States: a territorial history.
Immediately after Independence, the 13 colonies had to find a way to admit new territories to the Union.
Two ordinances, passed in 1787 and 1789, established that these territories would be placed under the authority of the Federal Government and that, once a territory’s population reached 60,000, it could submit a request to become a new State of the Union.
Before the end of the 18th century, three new States joined the original 13 States: Vermont, Kentucky and Tennessee.
At the start of the 19th century, two new states were admitted into the Union: Ohio in 1803 and Louisiana in 1812.
At that time, a major division along political and ideological lines emerged between the Northern States, which were gradually abolishing slavery, and the Southern States, where the striking development of the cotton plantations had strengthened the defenders of slavery.
In the Senate, political stability depended on the delicate balance between the number of free states and the number of slave states.
The next group of states to join the Union allowed the balance to remain in place: Indiana and Mississippi, followed by Illinois and Alabama.
However, Missouri’s request for admission as a slave state was strongly opposed by the abolitionists. The creation of Maine off the state of Massachusetts allowed the numbers to stay even.
Arkansas and Michigan were admitted to the Union in 1836 and 1837.
Florida and Texas as slave states in 1845, then Iowa and Wisconsin as free states in 1846 and 1848.
But now, western expansion was opening up lands that were ill-adapted to growing cotton and using slave labour, a situation which threatened the South’s position.
After much debate, in 1850 California joined the Union as a free state.
In Kansas, admission into the Union was delayed when violent clashes erupted between supporters and opponents of slavery.
The entry of Minnesota, in 1858, and later of Oregon the following year tipped the balance even further towards the free states.
Kansas finally joined as a free state in 1861. This served as a breaking point that, along with the election of Abraham Lincoln as President, led to the secession of 11 Southern States from the Union and the outbreak of the Civil War.
The first act of war was the bombardment of Fort Sumter in Southern Carolina, launching four long years of bloody conflict between the Union States and the Confederate States.