This thesis examines intelligence operations conducted by Major General Rosecrans' Army of the Cumberland during the initial phases of the Chickamauga Campaign (11 August to 16 September 1863). The thesis methodology is a detailed analysis of all intelligence reports received by the headquarters and a detailed examination of all outgoing correspondence from the headquarters intended to identify the analytical process used and the impact of intelligence on Rosecrans' decision making during the campaign. The record shows that contrary to popular historical opinion there was significant intelligence available indicating the probable Confederate course of action. General Rosecrans and his staff actively discounted information that did not conform to their pre-conceived expectation or template of the enemy with tragic results for the Army of the Cumberland. This thesis highlights several timeless lessons of relevance to the modern military officer: the importance of focused intelligence collection operations, the requirement for clear thinking and disciplined analysis of intelligence reporting, the dangers of over-confidence and preconceptions, the hazard of focusing on one's plan instead of the enemy, and the importance of avoiding "group-thinking" among a staff.