The Vietnam War has been subject of considerable research, both immediately after its conclusion and in more recent times in light of the new prolonged conflicts involving the United States armed forces. Yet, despite the considerable amount of published and unpublished material several assumptions have been accepted without the necessary criticism. One of these assumption is the fact that the US Army under General William C. Westmoreland was a static and unimaginative organization while the US Marine Corps had found the key to defeat the communist insurgency in Vietnam. The aim of this thesis is to examine this assumption in the context of the two services development before 1965 and of the conduct of their operations during the actual war. Examining the development of US counterinsurgency doctrine demonstrates that the US Army was not a passive spectator but took an active lead in the process. Furthermore there is no evidence that the US Marine Corps (USMC) was able to craft a war winning strategy in Vietnam and that its inability to operate in a combined arms and combined services environment damaged the overall effectiveness of the American war effort. These differences emerge from the fact that, contrarily to the common opinion, the USMC was the less flexible organization dominated by a close group of infantry officers while the Army, owing to its more complex make up, was able to operate with flexibility and efficiency crafting an effective method to fight in Vietnam.