Mississippi finally ratified the 13th Amendment in 1995.
Derleth’s original sentence was “The EP forced the issue of slavery to the fore and prevented Europe (England in particular) from helping the CSA.” In other words, the Emancipation Proclamation at least symbolically turned the Northern war effort from a war to preserve the Union into a war to end slavery, making it politically impossible for the European powers to intervene on the side of slavery.
Some maps of interest:
abolition map of 1800
abolition map of 1821
abolition map of 1840
abolition map of 1850
New Hampshire abolished slavery in 1783. Vermont was the first in 1777. The confusion about dates for some states results from a gradual transition from slave state to free. Here’s the list so far from various sources, mostly the maps referenced above. (* denotes gradual abolition):
1777: Vermont
1780: Massachusetts (and Maine)
1780: Pennsylvania* - a free state by 1850. read the 1780 Act for the Gradual Abolition of Slavery
1783: New Hampshire
1784: Rhode Island* - a free state by 1842
1784: Connecticut* - a free state by 1848
1787: Territories Northwest of Ohio River (Ohio, Indiana, Illinois, Michigan (Wisconsin))
1799: New York* - a free state by 1827
1846: New Jersey
1846: Iowa
1848: Oregon Territory
1850: California
1865: Maryland, Delaware, Missouri, Kentucky
1865: CSA
The Emanicpation Proclamation only applied to those specifically named states and territories in active rebellion against the Union. Slavery in all states, including those loyal to the Union, was finally abolished when the 13th amendment to the Constitution was ratified in 1865.