On Saturday, 133 years after the Confederacy was defeated by the Union, a memorial to those black soldiers is being dedicated in Washington.

The monument is being kept under wraps

The essence of the conflict was slavery and the southern states' refusal to accept its abolition, which led to a bloody civil war.

Thousands of runaway slaves, along with many free blacks living in the north, joined the Union army and fought with distinction.

Men such as First Sergeant Powhatan Beattie who, on 29 September 1864, took command of his company in a battle in Virginia and led it "gallantly" after all the officers had been killed.

But although the Union was fighting against slavery it was itself guilty of racism.

Black soldiers were paid less than their white comrades and at the end of the war black veterans were not invited to the victory parade.

Robert Young talks to two youngsters about the Civil War

Robert Young, whose ancestor fought in the war, says: "When I grew up studying history there was no mention of black soldiers at all.

"The memorial, to me, represents the beginning of a broader education for America and everyone to know that as a population of slaves they were willing to fight for their own freedom."

In recent years attempts to combat this ignorance have been boosted by literature and films such as Glory, starring Denzel Washington and Morgan Freeman.

Morgan Freeman as a volunteer in the film Glory

Today six million black Americans could trace back their family trees to Civil War veterans.

But historians and black activists feel it is not only blacks but whites too who would benefit from learning about their debt to the soldiers.

Carol Hector Harris, another descendant, says: "America itself needs to know about that history and not overlook that history because this country would not be where it is today if these men had not fought."

The vast majority fought for the Union but a smaller number were persuaded by their slave masters to "defend the Confederacy against invasion by the fanatical invader".