The theory and practice of blockade entered a third stage (following the formative period in the seventeenth and eighteenth centuries and the somnolent period dating from 1815) when the major nations of Europe joined in the Great War in 1914 and the United States entered the conflict in 1917. In the opening campaign along the Marne, the French army and a small British force slowed the German army, and the war turned from movement to position. By this time, the narrow definitions of the Paris and London declarations were all but obsolete—inoperable except in the most unimportant situations. Those formulations could not serve the needs of twentieth-century war. Transportation networks had grown up across all modern countries—rivers and canals, railroads, macadamized roads and highways for motor travel. New navies of steel and steam plied the seas, epitomized by the launching of the British battleship Dreadnought in 1906, and the conversion of merchant shipping from sail to steam was virtually complete. The new network of transportation made enemy ports of less consequence as goods could be shipped in and out via nearby neutrals. The conversion to steam made it unnecessary for warships to catch the wind before they moved out against blockade-runners or illicit neutral traders. Merchant shipping could operate apart from trade routes and prevailing winds, quite literally turning up anywhere. Then, too, the industrialization of much of Western Europe, especially of Germany, the principal antagonist of the Allies, gave a new dimension to blockade— the right sort of blockade might destroy German industry, or at least severely cripple it. And, as if blockade in the sense of Grotius and Bynkershoek would have had much chance at all, practical submarines and highly reliable mines were invented just prior to the outbreak of the war. Small wonder that World War I virtually ended blockade as it had been known.
In several instances, the old, traditional blockade of close-in surveillance of enemy ports was practiced during World War I, but these operations, carried out in reasonable conformity to the strictures of the London Declaration, occurred in secondary theaters of naval activity—the Mediterranean, Africa, and China. Where things really mattered—the blockade off the European coast and in the North Sea—the British government at least made a formal effort to maintain that the old rules still roughly applied. Its foreign secretary, Sir Edward Grey, indulged in frequent long explanations to the American ambassador in London and to the secretary of state in Washington. He assured them and reassured them that any changes in traditional rules of blockade stemmed not from a spirit of innovation but from the need to adjust old definitions to new circumstances. He implied that nothing really had changed.
In fact, the purposes of a blockade during the war were assisted by other measures not given that name, and the belligerents were usually careful in avoiding use of that term to describe their actions. The outstanding examples of such measures were the German declarations of war zones and the British designations of mine areas, military areas, and danger zones. It was a piquantly interesting fact that every one of the belligerent resorts to what might be described as measures of quasi-blockade was adopted via the principle of international law known as the law of retaliation.
It is hardly necessary to explain in detail how the British and German blockades operated during World War I. The Germans first used mines, giving notice of their intent to do so in August 1914; the British countered in November by closing the entire North Sea because of minefields. In a proclamation of 4 February 1915, the Germans announced unrestricted submarine and mine warfare, and the following month the British and French retaliated by declaring their intent to detain and take into port all ships "carrying goods of presumed enemy destination, ownership, or origin." To the Americans, Grey described that move as a blockade, and the U.S. government took his note as a formal notice of the establishment of a blockade and sought to get the Allies to respect neutral ports and vessels according to traditional standards of international law. In responding to the strictures of Washington officials, Grey took the stand that the March order amounted to "no more than an adaptation of the old principles of blockade." A decade later in his memoirs he wrote more candidly: "The Navy acted and the Foreign Office had to find the argument to support the action; it was anxious work."
And well done. The order in council of 11 March 1915 was one of the most powerful weapons in the British system of economic warfare against Germany. With the order the Allies gained all the advantages that would have come from a formal declaration of blockade of German ports—and without the inconvenience of stationing "adequate" naval forces off the ports. Also, when the Royal Navy acted under that mandate it succeeded in reducing the flow of supplies to the enemy via neutral ports, while at the same time cutting down on Germany's overseas trade. If the Allied pursuit of victory impinged too closely upon the interests of neutrals, if American shippers complained about innovations or illegalities, the lawyers at the Foreign Office had little trouble finding arguments.
Ironically, the British government found quite a few in the practices of the Union navy during the Civil War. One of the more interesting adjuncts to the British system of blockade closely paralleled a technique used by the North in its efforts to subdue the South, although there is no evidence that the British borrowed the idea from the Americans. From 1861 to 1865, Union officials sought to cut off supplies to the Confederacy by requiring exporters to obtain licenses, which, of course, could only be procured upon evidence that the cargoes in question had a bona fide neutral use. During World War I the British played a variation of this theme in order to make their restrictions more acceptable to neutrals. Their system, called navicerting, worked thusly: A neutral exporter, say an American interested in perfectly legal, noncontraband trade with a Scandinavian country, applied to British authorities (usually a consul) for permission to send his ship through naval cordons guarding approaches to European coasts. If he convinced the consul of the innocence of his venture, he received a navicert, a sort of commercial passport, which in most cases assured noninterference with his ship and cargo. The innovation conferred enormous advantage on the British, for in effect it transferred control of a large measure of neutral trade "from the deck to the dock."
The Allies introduced other refinements in their practice of blockade. They censored neutral mail to discover firms trading with the enemy and then blacklisted them and subjected them to harassment, such as depriving them of coal or denying them facilities for repairs. In the case of the Kim a British prize-court judge accepted statistical evidence to establish a presumption that American goods consigned to a Danish port were actually on a continuous voyage to Germany. Prize courts also made a slight but significant alteration in procedure when they transferred the burden of proving the innocence of a cargo to the claimant. With such pressures the British secured neutral cooperation with their system of economic control. The alternative, to be sure, was not to cooperate, but the Allied supremacy at sea meant that noncooperation had unpleasant consequences: detention of cargo, expensive and time-consuming legal proceedings, confiscation of goods, and the denial of facilities needed to conduct business. All the while the German blockade of Britain moved back and forth from observance of rules of cruiser warfare (meaning that submarines would not sink on sight but instead would visit and search and if necessary make provision for the safety of passengers and crew) to the waging of unrestricted submarine warfare, with its barbaric killing of noncombatants.
With a poor harvest in the United States in 1916 it appeared possible to prevent grain from other neutrals entering the British Isles, and in that hope the German government at the end of January 1917 announced unrestricted submarine warfare and knowingly brought the United States into the war, believing that their submarine blockade would bring victory before the Americans could raise and transport an army to Europe.
That the Allied long-range blockade of Germany was effective in helping the Allies win the war must admit of little doubt. Its effect upon the enemy, however, took far longer than expected. During the great German offensives in the spring and early summer of 1918, the troops were astonished at the quality of the equipment of the routed British Fifth Army, and also at the relative luxury in which French civilians were living. In the last weeks of the war the German home front virtually caved in. The military forces had been defeated first—there may be no doubt about that. But the civilians had been standing in line for years, and the word Ersatz had become a commonplace. When the prospect of peace emerged during the diplomatic exchanges of October 1918, the German civilians came to believe that they had suffered too long. The British blockade at last had placed an impossible burden. Continuation of the war into 1919, which otherwise might have been possible, was too harrowing to contemplate.
In examining the operation of the blockade during World War I, there remains the practice of the United States once it entered the war. Popular wisdom at the time and later had it that the Americans turned the principles of neutral rights around as soon as they became a belligerent. Such was not the case. There appear to have been only two clear-cut and rather minor violations of international law by the U.S. government. Use of embargoes, bunker control, and the blacklist were all unquestionably legal, involving only domestic jurisdiction and municipal law. The task of enforcing the blockade remained in British hands; the U.S. Navy took no prizes. It is true that it laid a great barrier of 70,000 mines from Scotland to Norway. During the period of neutrality, the United States had reserved its rights on British and German mining of the seas, and so, technically at least, did nothing startlingly novel by constructing a huge mine barrier in 1918. Fear of German use of Norwegian territorial waters disappeared when Norway closed the gap in the barrage by mining its own waters. In the pre-armistice negotiations with the British about their insistence upon reserving freedom of the seas from discussion at the peace conference (because they feared it might restrict the right of blockade) President Woodrow Wilson stated that blockade was "one of the many things which will require immediate redefinition in view of the many new circumstances of warfare developed by this war." But there was, he said, "no danger of its being abolished."
In the decades to come, "blockade" as a term would give way to terms such as "quarantine," "interdiction," and "interception." The belief was that the new world organizations, the League of Nations and, later, the United Nations—not to mention the Kellogg-Briand Pact (1928), by which almost all the nations of the world renounced and outlawed war—ensured that neutrality in the old sense was gone, perhaps replaced by collective security.
The record of the League of Nations in this regard was probably too short to be conclusive. Article 16 of the Covenant of the League of Nations provided for sanctions, and the first assembly of that organization created an International Blockade Committee, which issued a cautious report in 1921. The committee felt that "to pronounce an opinion in regard to the naval blockade and the right of search" was outside the scope of its work because more study was needed on those subjects. It noted that three great exporting countries (the Soviet Union, United States, and Germany) remained outside the league, and in a curious bit of logic concluded that its name was a misnomer, since application of Article 16 did not involve imposition of blockades in the traditional sense.
In the 1930s there was talk, in and out of the league, about quarantining aggressors. Membership in the league did not seem to make much difference in the results of the talk, for there were virtually no results. League members hesitated to include an oil embargo among the measures invoked against Mussolini's adventure in Ethiopia in 1935–1936, for the Duce said that he would consider an oil embargo an act of war. In 1937, President Franklin D. Roosevelt spoke in Chicago about quarantining aggressors, but he gingerly withdrew the idea when it aroused a storm of protest from his fellow citizens. In that troubled decade before the outbreak of World War II, there was plain evidence that neutral thoughts had not disappeared. In 1932 the United States ratified a Pan-American Convention that reaffirmed the belligerent right of visit and search. The neutrality acts (1935–1941), passed by Congress to keep the country out of war, went into great detail about neutral rights and duties, including those that involved blockades. The acts implied that for the United States the league's new order would not function. Nor did it.
During World War II the United States again was forced to confront the problems of blockade, both as a neutral and as a belligerent. While neutral, it took an indulgent stand in regard to British blockade measures and sought to avoid repetition of the arguments during World War I. The United States reserved its rights under international law but in practice reached a cozy accommodation with British restrictions, including a much expanded version of the navicert system. Then, as a belligerent, the United States found frequent occasion to expand the use of blockade as a weapon of victory. When the U.S. Navy achieved dominance in the Pacific, it instituted a tight cordon around the Japanese home islands to cut off imports and enforced that cordon with the chilling efficiency of its submarine fleet. Clearly, the demands of total war imposed a need for permutations of what international lawyers describe as the latent law of blockade—the belligerent that possesses command of the sea is entitled to deprive an opponent of use of it. Drafters of the Dutch Placaart of 1630 would have understood that need.
But what had been appropriate to the period of total war proved inappropriate to the era of limited wars that plagued the world after 1945. Once more the United States devised modifications in its use of blockade. These variations have been called "special function blockades" and had a wide use in the Korean and Vietnam wars, during the Cuban missile crisis of 1962, and in conflicts with unfriendly regimes in Iraq, Haiti, and the former Yugoslavia in the 1990s.
Because of its predominant naval power during the Korean conflict, the United States imposed a close-in blockade of all Korean ports and coasts that was reminiscent of the practice of earlier eras. In Vietnam, other tactics were used to interdict the flow of supplies to enemy contingents operating in South Vietnam. To prevent introduction of war matériel by sea, the South Vietnamese, with the aid of U.S. naval craft, conducted what might be described as a self-blockade. During this time they practiced all the traditional techniques of visit and search off their own coasts and in their own territorial waters. Perhaps a more interesting case was the mining of Haiphong Harbor and the internal waterways and coastal waters of North Vietnam by American aircraft in the spring of 1972. By this process of air interdiction of seaborne supplies the United States hoped to carry out the purposes of a blockade, and took extreme care to minimize the danger to neutral commerce. The U.S. Navy even observed the ancient custom of allowing a grace period for neutral vessels to leave the coast and harbors before activating the mines.
The most important invocation of a special function blockade occurred in the Caribbean during the missile crisis in October 1962. Thus, the United States relied on one of the oldest weapons in its arsenal to avoid resorting to more deadly ones. In a speech to the nation, President John F. Kennedy announced "a strict quarantine of all offensive military equipment under shipment to Cuba" and a turning back of all ships found to be carrying offensive weapons "from whatever nation or port." The subsequent proclamation ordered U.S. forces to interdict delivery of such matériel, and the navy set up a quarantine line around the eastern and southern approaches to the island. The president carefully coordinated his naval maneuvers so as to leave room for compliance by the Russians with these restrictions on their freedom of the seas. Successful resolution of the missile crisis again demonstrated the protean possibilities of concepts of blockade or "proceedings in the nature of a blockade," that is, as flexible instruments of national policy.
In the case of blockading—such in reality was the word behind the other words—of Iraq that began in August 1990, the action against Iraqi trade divided into phases I and II. Phase I came at the outset of Operation Desert Shield, the effort by the United States together with a large number of other members of the United Nations to persuade Iraq to give up its occupation of Kuwait, which it had just accomplished. The lead in this effort to make Iraq move its military forces back behind its own borders was necessarily taken by President George H. W. Bush, for it was mostly U.S. Navy forces that would have to handle restrictions on ocean-borne trade; 95 percent of Iraq's exports were oil. The president skirted mention to blockade, at first referring to interdiction, then to interception. His effort to avoid a possible act of war was due to the sensibilities of UN members, some of which—notably France and Russia—were not in favor of strong measures. The president also hesitated because of lack of full support by Congress. But diplomacy abroad and at home gradually resolved the fears of the president's critics. Only two pipelines exited Iraq, one to Turkey and the other to Saudi Arabia. The former nation was cooperative, as was the latter, with a border to Kuwait. The only local supporter of the Iraqi dictator, Saddam Hussein, was Jordan, the monarch of which, King Hussein, might have been expected to oppose Iraq, where Saddam's political party, the Baath, had murdered Hussein's cousin, King Faisal, years before. Jordan's trade with Iraq also was at stake. President Bush managed to persuade Hussein to support a naval interception. As the months passed, with U.S. forces and those of its allies gathering in preparation for Operation Desert Storm, Saddam managed to alienate even more UN members by defying, if only in words, the interception of tankers by the U.S. Navy. To no avail the Iraqi representative in the UN denounced what he described as a U.S.-dominated action, asserting it was naught but a blockade. Phase II of interception of Iraqi trade commenced after the triumphant few weeks of Desert Storm, and, to the confusion of U.S. and UN officials, continued into the twenty-first century. The basic problem during Phase II was to get the Iraqi regime to change its ways, for they were intolerable—a total disregard for human rights, support of international terrorism, hegemonic ambitions in the area, and development of weapons of mass destruction and missile delivery systems, not to mention refusal of reparations for its invasion of Kuwait. But how to force a reformation? Claims by Iraq that children suffered from lack of food were difficult to resist, and the UN was tempted to allow oil for food if only to pay Kuwait and nations such as Jordan, which had to house and feed refugees during Desert Shield and Desert Storm, as well as nations whose nationals had been trapped in Iraq during that time. Meanwhile, the U.S. Navy's interceptions were difficult to enforce. Ships bringing cargoes into the country, particularly container ships, had to make space for inspectors, and the loss of cargo thereby led to complaints. Ships destined for Jordan's port of Aqaba required inspection at sea or at the port prior to land transshipment. Sentiment arose, encouraged by Iraq, to hold inspections at a designated point on the Jordan-Iraq border, so that the Iraqis could boast about doing everything properly, while goods could come in at other border points illegally and without notice. (The borders in the area had no passes or notable roads; they were lines drawn in the sand after World War I.) In 1997 the commander of the U.S. Fifth Fleet accused Iran of complicity for smuggling oil through Iranian territorial waters. As late as March 2001 a notable breach in the interception was occurring by land transit of oil into Turkey, with the oil duly taxed by the Turkish government. Both the United States and United Nations informally accepted the traffic, amounting annually to hundreds of millions of dollars, for it benefited the weak Turkish economy.
Meanwhile, between 1991 and 1994 actions leading to interdiction were taken in Haiti, to force out a military regime that had deposed an elected president, and against Serbia for attacking Slovenia and Croatia, which had declared independence from Serbia. At the beginning in Haiti the international opposition was marshaled by the Organization of American States and was voluntary. At last, in 1993, the United Nations took over from the OAS and imposed mandatory prohibition on entry of oil, arms, police equipment, and "spare parts for the aforementioned." There nonetheless was trouble over border smuggling from the Dominican Republic, the government of which reluctantly allowed policing by U.S. military helicopters. On 19 September 1994, without any clear evidence that a limited embargo had had any effect, a small invasion force landed and the military regime came to an end.
In regard to Serbia, a situation developed not unlike that of the American Civil War, in which the Yugoslav federal government, dominated by Serbia, sought to bring back its independence-minded provinces. Two international squadrons stationed in the Adriatic undertook to enforce an embargo. But the government in Belgrade needed no arms support, unlike its seceding provinces. And other supplies to Serbia were easily available via the Danube or neighboring states. As in Haiti, naval measures of interception ultimately failed, and led to employment of air power and land-based peacekeeping forces.