Many theorists have discussed the question of the fundamental principles of warfare. But the importance of the principle of concentration of force for warfare is controversial and the theories about the principle are too general to say something about the reality. So what is required to achieve success with a concentration of force? In order to answer this question, this essay deals with the basic principle of warfare on the concentration of force from recognized theorist, Carl von Clausewitz, Sir B.H. Liddell Hart, John M. Collins and Milan Vego, in the context of submarine attacks. The study has a deductible theoretical research effort and the purpose is to identify and describe which variables that contribute to success in implementing a concentration of force and how they affect submarine attacks. To create a coherent image of the principle, the principle is measurable by identifying variables to achieve success with concentration of force. From the theories, the author identifies five variables: aggregate strength, spread, opponent knowledge, mobility and misleading. To investigate the principle, the variables are tested against two submarine attacks during World War II, which are known to be cases where the concentration of force was used. The result shows that during the two attacks only two variables, combined strength and spread were identified. During the attack on the HX 229/SC 122 convoys, which from the attacking party are considered a successful concentration of force, all variables were identified. However, the attack on the SC 130 convoy was considered as a failed concentration of force, since only two variables were identified.