They knew the terrain and could move within many areas without being noticed; their information represented a prolific and productive category of intelligence obtained and acted on by Union forces throughout the Civil War.
The value of the information that could be obtained, both passively and actively, by black Americans behind Confederate lines was clearly understood by most Union generals from early in the war.
Gen. Robert E. Lee, commander of the Confederate Army of Northern Virginia, was equally aware, and in May 1863 he said, "The chief source of information to the enemy is through our Negroes."
It is an omnipresent spy system, pointing out our valuable men to the enemy, revealing our positions, purposes, and resources, and yet acting so safely and secretly that there is no means to guard against it.
Even in the heart of our country, where our hold upon this secret espionage is firmest, it waits but the opening fire of the enemy's battle line to wake it, like a torpid serpent, into venomous activity.
Before fleeing Richmond in 1865, the Confederate Secretary of War ordered the destruction of virtually all intelligence files, including counter-intelligence records regarding Union spies.
He furnished intelligence on Confederate fortifications and troop movements to Gen. Benjamin F. Butler, commander of Fort Monroe located at the mouth of the James River on the tip of the Virginia Peninsula.
Based on the intelligence gained from these missions, Butler determined that Confederate forces were planning an attack on Newport News, capture of which would isolate Fort Monroe from Union resupply.
As the number of Union forces increased, requiring better organization, Maj. Gen. George B. McClellan was appointed as commander of the Army of the Potomac defending Washington.
While each group provided valuable information, Pinkerton soon discovered that the former slaves were the most willing to cooperate and often had the best knowledge of Confederate fortifications, camps, and supply points.
Scobell often used his membership in the "Legal League," a clandestine Negro organization in the South supporting freedom for slaves, to acquire local information.
Mary Louvestre, a freed slave, worked in Norfolk as a housekeeper for an engineer who was involved in the refitting and transformation of the USS Merrimack into the Virginia, the first Confederate ironclad warship.
Overhearing the engineer talking about the importance of his project, she recognized the danger this new type of ship represented to the Union navy blockading Norfolk.
In March 1862, Robert Smalls, a free black, rowed out to a Union warship that was part of a large fleet assembled to attack the seacoast town of Fernandina, Florida.
Based on the intelligence he provided, Union forces attacked Fernandina and routed the enemy's rear guard before the Confederates could sabotage the harbor.
On the night of May 12, 1862, he, family members, and other black American crewmen of the Planter, a cargo steamer turned into an armed coastal patrol ship, sailed out of Charleston harbor after the captain and two mates had gone home.
On 11 February, Col. George H. Sharpe, an attorney and an officer of New York state volunteers, accepted the post of head of the Army's intelligence service.
Sharpe obtained, collated, analyzed, and provided reports based on scouting, spying behind enemy lines, interrogations, cavalry reconnaissance, balloon observation, Signal Corps observation, flag signal and telegraph intercepts, captured Confederate documents and mail, southern newspapers, and intelligence reporting from subordinate military units.
Sharpe's BMI was well established when Charlie Wright, a young black man, arrived at Union lines from Culpeper, Virginia, in June 1863.
John McEntee, an officer from the BMI who had deployed with Union cavalry forces just after the battle of Brandy Station, telegraphed Sharpe the following: "A contraband captured last Tuesday states that he had been living at Culpeper C. H. for some time past.
Hooker ordered his army to shadow the Confederate forces' movements while traveling on the eastern side of the Blue Ridge Mountains out of view of Lee's troops.
While many reasons can be cited for Lee's defeat at Gettysburg, there can be no doubt that the ground held by the Union forces played a significant role in the victory.
Because both sides were poorly prepared for the war, notwithstanding the many years of political buildup to the actual fighting, there apparently were few intelligence agents who had been specifically placed in the enemy's institutions.
While no record remains of the specific intelligence he produced, it apparently was valuable enough to cause General McDowell to telegraph it immediately to the War Department in Washington.
When Van Lew decided to establish a spy ring in Richmond shortly before the fighting began, she asked Bowser to return and work with her for the Union.
Van Lew obtained a position for Bowser as a servant in the Confederate "White House" through the recommendation of a "friend" who provided supplies to that household.
She reported her intelligence to Col. James Montgomery, a Union officer commanding the Second South Carolina Volunteers, a black unit involved in guerrilla warfare activities.
In late May, Gen. David Hunter, commander of all Union forces in the area, asked Tubman to personally guide a raiding party up the river.
The story involved a runaway slave named Dabney, who crossed into Union lines with his wife and found employment in General Hooker's headquarters camp.
Dabney explained that he and his wife had worked out a signaling system based on the laundry that she hung out to dry at her mistress's house, which was observable from Hooker's headquarters.