Русско-англо-русский юридический словарь

In U.S. criminal law, necessity may be either a possible justification or an exculpation for breaking the law. Defendants seeking to rely on this defense argue that they should not be held liable for their actions as a crime because their conduct was necessary to prevent some greater harm and when that conduct is not excused under some other more specific provision of law such as self defense. (Wikipedia)

Necessity: A defense that permits a person to act in a criminal manner when an emergency situation, not of the person's own creation compels the person to act in a criminal manner to avoid greater harm from occurring. (http://nationalparalegal.edu)

The armed forces of a country are its government-sponsored defense, fighting forces, and organizations. They exist to further the foreign and domestic policies of their governing body and to defend that body and the nation it represents from external and internal aggressors. In broad usage, the terms "armed forces" and "military" are often treated synonymously, although in technical usage a distinction is sometimes made in which a country's armed forces may include both its military and other paramilitary forces. (Wikipedia)

Civil defense, civil defence or civil protection is an effort to protect the citizens of a state (generally non-combatants) from military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation and recovery. (Wikipedia)