Under the traditional concept of blockade, a belligerent was entitled to proclaim a blockade of all or part of the enemy’s coast and to use warships to enforce that blockade. There was no legal obligation to comply with a blockade, but any merchant ship, whether belligerent or neutral, that was intercepted by the blockading State while attempting to run the blockade was liable to capture. Following the decision of a prize court, ship and cargo became the property of the blockading power. The traditional concept of the blockade was confined, therefore, to the law of naval warfare. Blockade today is a technical legal term that most people, including lawyers, use without much precision to describe a variety of conduct beyond maritime operations.

In order to be lawful, a naval blockade had to be formally declared and had to be effective, that is to say it had to be properly enforced by warships of the blockading State. At one time this meant stationing warships just off the coast of the State that was subject to blockade, but by the time of the two World Wars, blockades were often administered from a long distance. An effective blockade allowed a belligerent to cut off all maritime commerce between its enemy and the rest of the world. The purpose was not only to prevent goods from reaching the enemy (to a large extent that could be done without a blockade in any event) but also to prevent the enemy from exporting to the outside world and thereby sustaining its war economy. Blockades were used to great effect during the American Civil War and the two World Wars, but since 1945 there have been very few instances of anything that could properly be described as a blockade in this technical sense.

Today, however, the term blockade is frequently applied to maritime operations undertaken at the behest of the United Nations Security Council. The council has on several occasions authorized warships to intercept shipping suspected of violating economic sanctions. Following Iraq’s invasion of Kuwait in 1990, for example, Resolution 661 imposed a ban on imports to, or exports from, Iraq and occupied Kuwait. Shortly afterward, Resolution 665 authorized states with naval forces in the region cooperating with the government of Kuwait to intercept shipping suspected of violating these sanctions. The result was in many ways very similar to that of a wartime blockade. Warships from several navies intercepted over ten thousand vessels between the summer of 1990 and the end of hostilities, and all maritime trade with Iraq and Kuwait was effectively cut off.

Nevertheless, operations of this kind differ from the traditional blockade in a number of ways. First, there is a legal duty resulting from the Security Council resolutions not to engage in sanctions breaking and any form of sanctions breaking is likely to lead to penalties. Second, the obligation to comply with the UN sanctions resolutions prevails over existing contracts and international agreements regarding shipping. Third, warships policing UN embargo operations of this kind are entitled to stop and search merchant shipping and to turn back ships suspected of sanctions breaking, but there are no provisions for the capture of ships or for prize court proceedings. For example, the owners of a Greek ship stopped by an American warship while trying to enter a Yugoslav port during the Yugoslav sanctions would probably be liable to penalties in the Greek courts but not in those of the United States. Finally, the imposition of a traditional blockade was generally regarded as an act of war, whereas enforcement of United Nations sanctions is a rather different act. Although sometimes there may be an armed conflict in place between the States whose warships are used to enforce sanctions and the target State (as was clearly the case in the Gulf War in 1991, and perhaps in the period leading up to it), that will not always be so. The enforcement of legal sanctions even by means of maritime embargoes is not necessarily an act of war.

In recent times, some commentators have also used the term blockade to describe land-based operations designed to cut off supplies to a particular town or area. These operations, which are more like the sieges of the past, are not really blockades at all. The large body of law relating to naval blockades is not applicable to them, there is no provision for the confiscation of property from those seeking to supply the besieged area, and the emphasis is on stopping supplies from getting in, rather than cutting off exports. There is also, as a result of Additional Protocol I to the Geneva Conventions, an obligation not to deprive the civilian population of the basic means of survival. The effect of this rule is far less clear-cut in naval operations, although a blockade that had as its sole purpose depriving the enemy population of food and other humanitarian supplies would be unlawful today.

(See humanitarian aid, blocking of; starvation.)