Better intelligence pays off for North

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Historians maintain that Gen. Robert E. Lee and the Army of Northern Virginia lost the Battle of Gettysburg in large part because of the lack of an effective intelligence-gathering capability. This was due to the separation of the cavalry under Maj. Gen. J.E.B. Stuart during the campaign.

What has received less attention is the extensive intelligence apparatus that the Union army had available to help it defeat the Rebel army.

After the Army of the Potomac's defeat at Chancellorsville in May 1863, Union Maj. Gen. Joseph Hooker knew that he had little time to prepare for another engagement with Lee's army. For this purpose, he needed to determine the strength, disposition and intentions of the enemy. At the same time, he wanted to ensure that Gen. Lee learned as little as possible about the plans and operations of the Army of the Potomac.

To accomplish these objectives, Hooker had considerable resources in place, including the provost marshal department and its recently established Bureau of Military Information, the Cavalry Corps, the Signal Corps, the U.S. Military Telegraph, special units, and spy networks.

Provost marshal

Gen. Marsena Patrick, provost marshal of the Army of the Potomac, was responsible for military intelligence and counterintelligence operations. These duties included directing a team of scouts and spies. He also interrogated Confederate prisoners of war and deserters, as well as escaped slaves or "contrabands" to gather information about the enemy.

Scouts conducted reconnaissance, and spies operated behind enemy lines. Patrick's counterintelligence duties included tracking down spies within Union lines and local citizens who served as guides for enemy raids and incursions.

When Hooker took command of the Army of the Potomac in early 1863, he ordered Patrick to establish a military intelligence unit. Known as the Bureau of Military Information (BMI), its mission was to collect information about the enemy derived from all available sources, evaluate and process this data, and report the resulting intelligence to the army commander.

The bureau

Gen. Patrick chose veteran regimental commander Col. George H. Sharpe, a veteran regimental commander, to direct the bureau, and put him in charge of collecting and analyzing data about the enemy's strength, disposition, movement and morale. The 35-year-old Sharpe was a Yale-trained lawyer from the Hudson River town of Kingston, N.Y. Sharpe had two capable assistants, John C. Babcock, a civilian, and Capt. John McEntee.