“Military necessity justifies a resort all the measures which are indispensable for securing this object and which are not forbidden by the modern laws and customs of war.” (US Army, 1914, p.14).

What military necessity admits of (General Order 100, 1864, Article 16):

• Military necessity admits of all direct destruction of life or limb of armed enemies, and of other persons whose destruction is incidentally unavoidable in the armed contests of war;
• It allows of the capturing of every armed enemy, and of every enemy of importance to the hostile government, or of peculiar danger to the captor;
• It allows of all destruction of property, and obstruction of ways and channels of traffic, travel, or communication, and of all withholding of sustenance or means of life from the enemy;
• Of the appropriation of whatever the enemy’s country affords that is necessary for the subsistence and safety of the army, and of such deception as does not involve the breaking of good faith, either positively pledged, regarding agreements entered into during the war, or supposed by the modern law of war to exist.

What military necessity does not admit of (General Order 100, 1863, Article 16):

• Military necessity does not admit of cruelty – that is, the infliction of suffering for the sake of suffering or for revenge, nor of maiming or wounding except in fight, nor of torture to extort confessions.
• It does not admit of the use of poison in any way, nor of the wanton devastation of a district.
• It admits of deception, but disclaims acts of perfidy; and
• In general, military necessity does not include any act of hostility which makes the return to peace unnecessarily difficult.

Reference

US Army. (1914) Rules of Land Warfare. Available from World Wide Web: http://www.loc.gov/rr/frd/Military_Law/pdf/rules_warfare-1914.pdf. [Accessed: 28 September, 2015].