History of North America

Why do many US Civil War historians believe was the most substantial military effect of Emancipation Proclamation?

A weakened southern economy:

The Emancipation Proclamation had a profound effect on the Southern economy. By depriving plantation owners of their unpaid workforce, it undermined the South's ability to produce cotton and other commodities for export. This, in turn, decreased the South's financial resources and made it more difficult for the Confederate government to fund its military operations.

Additionally, many African Americans who had previously worked on plantations in the South escaped to the Union Army, where they served as soldiers and laborers. This further weakened the Southern economy by depriving it of valuable manpower.