The declaration of the Emancipation proclamation by then President Abraham Lincoln, extinguished any hopes of Foreign assistance to the Confederacy. Both the nations of Britain and France had long ago abolished slavery, and any hopes of support from the European powers withered away in conjunction with this proclamation. This also saw the addition of black recruits within the ranks of the Union Army, understanding that a military victory over the south would be the only way of ensuring freedom.

The use of the 13th Ammendment (Emmancipation Proclamation) as a tool to further isolate the South during the Civil War, was also the strategy to eliminate the inmediate reason of the war.

Explanation:

The final defeat of the Confederacy can be explained by its failure to procur an international recognition. And one of the key factors in this was the slavery issue. European powers, that had abolished slavery between 1830-1850, did not wanted to be associated to it, but weighted their options until the Union blockade of the South ports, and military victories seemed to define the course of war.

This was also fostered by the injection of recruits of African-American origin into the Union's army such as the Massachussets 54th regiment (portayed in the film "Glory").