Military doctrine is the definition of how military forces contribute to campaigns, battles, engagements and major operations. Military doctrine is a formal expression of military knowledge and thought. Doctrine is not a strategy, it is presenting the manner in which army power should be developed and applied it to achieve national objectives or those of a group of nations globally. According to NATO the definition of Doctrine is explained as "Fundamental principles by which the military forces guide their actions in support of objectives. It's authoritative but requires judgement in application" which was accepted by many nations unaltered. Military doctrine is the expression of how military forces contribute to campaigns, major operations, battles, and engagements. It is a guide to action, rather than hard and fast rules. Doctrine provides a common frame of reference across the military. The purpose of the doctrine is to ensure war is morally justifiable through a series of criteria, all of which must be met for a war to be considered just.