Groves selected Oppenheimer to head the project's secret weapons laboratory, although it is not known precisely when. This decision surprised many, because Oppenheimer had left-wing political views and no record as a leader of large projects. Groves worried that because Oppenheimer did not have a Nobel Prize, he might not have had the prestige to direct fellow scientists, but Groves was impressed by Oppenheimer's singular grasp of the practical aspects of the project and by the breadth of his knowledge. As a military engineer, Groves knew that this would be vital in an interdisciplinary project that would involve not just physics but also chemistry, metallurgy, ordnance, and engineering. Groves also detected in Oppenheimer something that many others did not, an "overweening ambition", which Groves reckoned would supply the drive necessary to push the project to a successful conclusion. Oppenheimer's past associations were not overlooked, but on July 20, 1943, Groves directed that he receive a security clearance "without delay irrespective of the information which you have concerning Mr Oppenheimer. He is absolutely essential to the project." Rabi considered Oppenheimer's appointment "a real stroke of genius on the part of General Groves, who was not generally considered to be a genius".