Martial Law is the exercise of arbitrary power by the supreme authority in a district or country where the ordinary administration has ceased to be operative, either on account of civil disturbance or because of the presence therein of a hostile force, though, in the latter case, the country would be more correctly described as being governed by the 'Laws of War.' Martial law was formerly synonymous with military law, and is often still confounded with it, perhaps because in the above-mentioned circumstances the supreme authority often avails himself of courts-martial and of the troops under his command to maintain order. Military Law (q.v.) is the law contained in the Army Act of 1881, which governs the soldier at all times, but affects civilians only when accompanying a force on active service; while martial law has been defined as 'no law,' but simply the will of the supreme authority. It is not recognised by British jurisprudence, and no rules are laid down for its application. It is assumed that, when the ordinary civil tribunals fail, the supreme authority will do his best to maintain order. He may therefore, if he thinks right, announce his intention of treating the civil population as though under military law, or in any other way that commends itself to him; but if they are British subjects he will have afterwards to justify his action by showing that it was absolutely necessary, and so obtain an indemnity from parliament for conduct which is in itself illegal. Military tribunals have several times been given power by Act of Parliament to try offenders against the public peace in Ireland, as in 1798, but the proclamations on these occasions merely justified the use of arms against rebellious subjects, not against peaceable citizens. They were announcements of the existence of a state of things in which force would be used against wrong-doers for the protection of the public peace, and were always followed by Acts of Indemnity.
On the Continent the practice is different, and when necessary a 'state of siege' is proclaimed in the disturbed district or occupied territory, and the inhabitants are thereby brought to a certain extent under military law.