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 elect a convention for drafting a Constitution and submitting a request to become a new State of the Union.
Already before the end of the 18th century, three new States joined the original 13 States:
Vermont, incorporating territories in western New Hampshire and eastern New York State;
Kentucky, previously part of Virginia ; and Tennessee, consisting of territories originally belonging to North Carolina.
Among the provisions of the Ordinance of 1787 was a ban on slavery in the North-West territories. This led to Ohio’s being admitted as a free state in 1803, followed by statehood being granted to Louisiana in 1812 as a slave state.
A major division, along political and ideological lines, emerged between the Northern States, which were gradually abolishing slavery, and the Southern States, where the spectacular development of the cotton plantations had strengthened the defenders of slavery. In the Senate, endless debates and compromises threatened to upset the delicate balance between free states and slavery 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. A compromise was found: Maine was created as a state without slaves with parts of Massachusetts, and a line traced along the 36°30’ would establish the boundary between future northern free states and southern slave states.
Arkansas and Michigan were admitted to the Union in 1836 and 1837.
Florida and Texas in 1845, then Iowa and Wisconsin in 1846 and 1848.
But now, western expansion was opening up lands that were ill-adapted to large plantations dedicated to cotton and using slave labour, a situation which threatened the South’s position.
California’s request for admission sparked new difficulties. It joined the Union as a free state in 1850.
In 1854, the Kansas-Nebraska Agreement stated that inhabitants of new territories were free to choose or reject slavery, thus bringing an end to the boundary set up in 1820.
However, increasing tension in Kansas led to bloody riots between partisans and opponents of slavery and led to delays in its admission.
The entry of Minnesota, in 1858, and later of Oregon the following year tipped the balance even further towards the free states.
Kansas finally became a State of the Union in 1861. This was the final straw which, together with the election of Abraham Lincoln as President, pushed the Southern States to secede and led to Civil War.