Final answer:
Frederick Douglass's pivotal contribution during the Civil War was helping African Americans enlist in the Union army, significantly impacting both the war and the future of civil rights.
Step-by-step explanation:
The main answer that best describes a contribution of Frederick Douglass during the Civil War is that Douglass helped African Americans enlist in the Union army. As a leading abolitionist, Douglass recognized the importance of African American service in the fight against slavery. He leveraged his powerful oratory and respected position to challenge free Blacks in the North to take up arms, encapsulated by his quote, "Who would be free themselves must strike the blow." Douglass's work was pivotal in encouraging close to 200,000 black men, both former slaves and freeborn, to join all-black Union Army regiments. This effort not only influenced the course of the war but also helped redefine the war's meaning, shifting it from a battle for union to one of emancipation and freedom.In conclusion, Frederick Douglass played a crucial role in rallying African American men to the Union cause, consequently impacting the outcome of the Civil War and the future of American society.