Military intelligence
When Major General Joseph Hooker took command of the Army of the Potomac he ordered Colonel George H. Sharpe to establish an intelligence unit. In 1863, they established the Bureau of Military Information. The information they gathered was amazingly accurate. They calculated the size of the Confederate Army of Northern Virginia within 2% of the actual number. By war's end Sharpe knew more about the Army of Northern Virginia than many Confederate officers. One of the best sources of military information Sharpe's organization had was slaves who used the Underground Railroad to escape the South.