Since the Lieber Code, the core meaning of military necessity is that the use of armed force and the destruction of life or properties which are not necessary to achieve military goals are prohibited.
In modern international humanitarian law, that does not imply, however, the admissibility of every conduct anyhow related to military goals. One has to recall that military necessity is framed in a different form in each norm of humanitarian law and is always conceived as an exception to prohibitive norms. Necessity cannot be considered an authorising principle detached from single provisions. Every provision is the outcome of a balancing process between the two conflicting values lying at the very basis of humanitarian law: humanity and military concerns. Every norm incorporates a peculiar equilibrium - which no external source can change - between those conflicting forces.
Military necessity can be defined as a principle underlying humanitarian law in that all its provisions are drafted taking into account its existence. Nevertheless the balancing of the opposing values is different according to the norms that come into consideration. The scope of the admitted derogation varies with regards to elements such as the degree of the necessity required, the nature of the circumstances from which the necessity arises, and the objective pursued in derogating to the prohibition.
Cf., for instance, articles 53 par. 2 (IV GC); art. 64 par. 2 (IV GC); Regulation 23 (g) HR.
Reference
Carnahan, Lincoln, Lieber and the Laws of War: The Origins and Limits of the Principle of Military Necessity, in American Journal of International Law, 1998, pp. 213-231, at 215.
Dinstein, Military Necessity, in Bernhardt (ed.), Encyclopaedia of Public International Law, Amsterdam, 1981-1990, pp. 395-397.
Downey, The Law of War and Military Necessity, in American Journal of International Law, 1953, pp. 251-262.
Greenwood, Historical Development and Legal Basis, in Fleck (ed.), Handbook of Humanitarian Law in Armed Conflict, Oxford, 1995, pp. 130-132.
Jaworski, "Military Necessity" and "Civilian Immunity": Where is the Balance?, in Chinese Journal of International Law, 2003, pp. 175-206.
Venturini, Necessità e proporzionalità nell'uso della forza militare in diritto internazionale, Milano, 1988.