Why did Sadao feel that the general was in the palm of his hand?
It means that Dr. Sadao understood that the General was in his control, as the latter thought it was due to his own carelessness that the American man had escaped unhurt in spite of his promise to Dr. Sadao about killing the man. The General now feared that this might be blamed on lack of patriotism or dereliction of duty on his part and thus, asked for Dr. Sadao’s loyalty.