Censorship in war-time seeks to withhold military information from the enemy, and to safeguard public morale from the corrosion of enemy propaganda. To achieve the first aim, press censorship is co-ordinated with censorship of radio, the mails, telephone, telegraph, and cable lines; and with appropriate safeguards against public access to prohibited areas.

The principles of military censorship are simple. The essence of successful warfare is secrecy. “Let us learn what we can from the enemy; let us teach him only what we must,” was the rule laid down by Lord Balfour in the last war. Surprise is still the most valuable “secret weapon” of war; and the ideal state of affairs from the narrow viewpoint of military operations is a complete black-out on all information regarding such matters as the strength of military forces, the disposition of units, the nature of defences, the stocks of war supplies, the rate of growth of the armed forces, the rate of production of war weapons, and so on. In modern wars, these desirable military secrets must be extended to facts concerning the economic, financial, and psychological strength of the nation at war.

The ideal state of affairs from the viewpoint of military operations is, unfortunately, at variance with other vital considerations. It is not practicable to screen all information from the enemy without hiding it quite as effectively from one's own people. Complete acceptance of the military philosophy of censorship would raise grave problems in a democracy, which relies so largely on an informed public, and on voluntary effort.