The principles of war are often used to describe the essential parts of warfare. There is however a discussion regarding what purpose they are supposed to have, what role they shall play or even how many principles there are. The military theorist Antoine-Henri Jomini stated that there was one fundamental principle: the principle of concentration of forces. Is this however true, and is Jomini’s theory still valid? The purpose of the thesis is to examine whether his theory is valid on cases from after Jomini’s career and to examine whether the principle of concentration of forces should be considered as one of the principles of war. The theory is tested on the Ardennes offensive in 1940, and Operation Overlord in 1944. The result of the examination shows that concentration of forces was one of the reasons that the examined cases were successful, but not the only one. As a result of this the theory that Jomini stated may be regarded as partly, but not fully valid. It also indicates that the principle of concentration of forces very well could qualify to be considered as one of the principles of war.