Complex sentence: 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, and became convinced that Oppenheimer was the best and only man to run the laboratory.
Split into simpler sentences: 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. Groves became convinced that Oppenheimer was the best and only man to run the laboratory.