The law of blockade, that is, the rules governing proper legal practice, originated in the early struggles for supremacy among the maritime nations of Europe in the fifteenth and sixteenth centuries. During that time, belligerents hacked and hewed, by sea as by land, and neutrals constantly found themselves involved in quarrels, whether they wished to be or not. Early in the seventeenth century a compromise of sorts emerged between neutrals and belligerents, in which the latter undertook to define carefully the list of items that were contraband and subject to capture. They also agreed that blockades could not merely be proclaimed: acceptable practice required that a port be cordoned off by the stationing of naval vessels at its entrance. As originally conceived, a blockade was a maritime equivalent of a land siege. When a port was properly blockaded, an investing belligerent could prohibit all trade with that port, including that of neutral nations. The idea apparently appeared first in a 1614 treaty between the Netherlands and Sweden. A refinement of the concept of effective blockade appeared in a Dutch announcement or Placaart of 1630, issued after consultation with private jurists and judges of the courts of admiralty. Its first article declared: "Neutral ships and goods passing in or out of the ports of the enemy in Flanders; or being so near them, that there can be no doubt but they will go into them, shall be confiscated: Because their High Mightinesses continually beset those ports with ships of war, in order to hinder any commerce with the enemy." Interestingly enough, the drafters of this rule justified it as "an ancient custom, warranted by the example of all princes"—a useful, if not entirely accurate, assessment of prior practice. The "law" of blockade, however, unlike other branches of international law, owed less to statutory enactments and more to the customs and precedents of naval officers and admiralty lawyers as they sought to bend the definitions of blockade to accommodate national interests, especially the need for victory in war.
In the early modern period of European history, with its frequent maritime wars, new rules of blockade rapidly evolved, and as they grew they acquired increasing importance as effective instruments of naval coercion. But those rules never remained static. They required frequent adjustment to new circumstances, technological innovation, and modified strategic concepts. More importantly, over the centuries, blockades had to be adapted to the exigencies created by new definitions of the nature of war.
From the mid-seventeenth century onward, new concepts of blockade were developed, and soon the nations of Europe agreed on some basic rules governing their use. Hugo Grotius, the father of international law, set out a rule that foodstuffs, so-called provisions, should be treated as contraband only when an attempt was made to introduce them into a blockaded port in extremis, and this humane refinement received general approval from naval authorities. For their part, legal theorists such as Cornelis van Bynkershoek generally agreed that international law recognized no right of access to ports effectively closed by naval squadrons. When the United States drew up a model treaty of commerce in 1776 for submission to foreign nations, it overlooked an article defining blockades, but soon remedied that omission. American statesmen took inspiration from the attempted Armed Neutrality of 1780. Catherine the Great sought to bring together some of the European neutrals in the general war then raging, to unite them in a pact of armed neutrality that would enforce an expanded definition of neutral rights in wartime. Russia proposed that "the denomination of a blockaded port is to be given only to one which has the enemy vessels stationed sufficiently near to cause an evident danger to the attempt to enter." Although little came of Catherine's initiative, the government of the United States incorporated this proviso into its Treaty Plan of 1784, and it sought international recognition of this principle.
In the seventeenth and eighteenth centuries, the practices of nations vis-à-vis blockade tended to follow their treaty obligations, and those treaties spelled out a wide variety of reciprocal rights and duties that would become operative in time of war. Naturally, a good many of the arrangements concerned the proper implementation of the rules of blockade, for nations that were neutral had no wish to become embroiled in the quarrels of their neighbors. It might be said that no branch of international relations received more attention than the search for a viable definition of blockade, one that would protect the rights of neutrals without too seriously impeding the war efforts of belligerents. Part of the reason for a certain tolerance on the subject stemmed from necessity: the naval powers of that day, Britain and Holland, depended upon Scandinavian sources of naval stores. Prudence dictated a circumspect policy toward the northern neutrals of Europe, while the conditions of warfare made such a policy easier to pursue.
In that time of limited war, full-scale blockades were rarely imposed, for men-of-war and privateers usually found it more profitable to waylay merchantmen that might be subject to seizure and condemnation in a prize court (with consequent enrichment of the captors). In prize law, problems of blockade violation remained largely a subdivision of the law of contraband until the era of the wars of the French Revolution. In the 1790s, British Admiralty officials began to pay closer attention to the problems posed by blockade. The altered circumstances of the time required new approaches, and all concessions toward neutrals had to be reevaluated.
The struggle that engulfed Europe from 1793 to 1815 ushered in a new era of international relations. Changed conditions of warfare required the belligerents to impose heavy restrictions on trade with the enemy. Almost at once, Great Britain and France narrowed their definitions of neutral rights; and as the struggle between them intensified, both nations demonstrated that they would take whatever measures seemed necessary to defeat the enemy. At one point the French proclaimed that there were no neutrals, and the British echoed that sentiment. According to one commentator, international law, if it existed at all, had been known only "through the declamations of publicists and its violation by governments." Whatever the cynicism of that mot, it accurately reflected the views of an age caught up in revolutionary upheaval.
When the wars of the French Revolution led Britain to an assault on America's presumed right of unfettered trade with all the nations of the world, belligerent as well as neutral, Secretary of State Thomas Jefferson drew up a strong protest. The provision order of 1793 had instructed British naval commanders to bring in for preemptive purchase all neutral ships en route to French ports with cargoes of corn, flour, or meal. By this arbitrary redefinition of contraband, by an order that would keep American grain out of French markets in Europe and in the West Indies, by a decree that arrogantly restricted American produce to the ports of Britain or its allies, the infamous provision order threatened the new nation's honor and interests. The threat led Jefferson to a spirited defense of America's canons of commerce and international law. After denouncing the British order as contrary to the law of nations and asserting that food could never be classified as contraband, he acknowledged a "single restriction" on the right of neutrals to use the seas freely: "that of not furnishing to either party implements merely of war … nor anything whatever to a place blockaded by its enemy." Jefferson thus put his finger on an important point, for if food could be classified as contraband, as an implement of war, British cruisers could legally seize it on American ships. If an order in council could declare entire islands of the West Indies or the entire coast of France blockaded, then Americans could carry nothing whatever to those places.
In the practice of the times, a legal blockade "entitled the blockading power to intercept all commerce with the blockaded port and to confiscate ships and cargoes of whatever description attempting to breach the blockade." As a nation that lived largely by export of foodstuffs, America had a vested interest in the outcome of arguments on the fine points of international law.
But Jefferson could not bring the British around to his view. Nor could John Jay when, in 1794, he went to London to draw up a commercial treaty and to resolve a number of simmering disputes, including claims for damages that had grown out of the British attacks on American commerce. In part, Jay sought to bring the British around to Jefferson's definition of neutral rights, to get them to agree that foodstuffs could never be classified as contraband (although they might be captured on ships attempting to enter a blockaded port). In these negotiations the Americans also desired British assent to the definition of effective blockade incorporated in the Armed Neutrality of 1780. Unable to obtain these arrangements, Jay had to be satisfied with a British promise to indemnify American citizens for captured articles "not generally regarded as contraband," and for assurances that vessels approaching a blockaded port would be turned away rather than captured, if the captain had no knowledge of such blockade. Beyond these innocuous concessions the British refused to go.
Meanwhile, on the French side during the 1790s, matters became so trying for the United States that a quasi-war broke out in 1798, largely because of French interference with American commerce. Lasting two years, the war ended with the Treaty of Mortefontaine (better known as the Convention of 1800), which contained a Napoleonic affirmation of neutral rights, including a narrow definition of blockade. No altruism dictated such a concession; in all probability Napoleon sought to embarrass the British for their provision order and for other executive acts of the British cabinet that both interfered with his supplies and irritated neutrals by their arbitrary nature. Or he may have sought to lure the United States into another league of armed neutrals that was then forming in Europe.
The Treaty of Amiens (1802) momentarily brought peace to Europe, but when war resumed barely a year later, the concept of neutral rights and the definition of blockade again came in for heavy pummeling by both belligerents. Horatio Nelson's victory at Trafalgar in October 1805 and Napoleon's at Austerlitz in December of that year made England supreme on the sea and France supreme on land. As Napoleon moved from triumph to triumph thereafter and consolidated his hegemony over the Continent, the British sought to bring him down with ever more restrictive maritime regulations. The military stalemate required full-scale economic war, which spelled trouble for the neutrals. The British had already tightened up on neutral trade with French colonial possessions by invoking the Rule of 1756—a diktat that forbade in wartime trade not allowed in peacetime—to cut neutrals out of the profitable French carrying trade.
During the wars of Napoleon, blockade proved the most potent weapon in the arsenals of both belligerents, although sometimes its bark was worse than its bite. Upon becoming prime minister in 1806, Charles James Fox sponsored an order in council that declared the coast of Europe, from Brest to the Elbe, in a state of blockade (although its prohibitions were absolute only between the Seine and Ostend). It amounted to a paper blockade, unsupported by ships stationed off the ports in blockade. Even the mistress of the seas did not have sufficient ships to cordon off so extensive a portion of seacoast.
The French responded with the Berlin (1806) and Milan (1807) decrees. These imperial enactments placed the British Isles in a state of blockade, and any ship submitting to search by British cruisers or complying with regulations requiring a stop at a British port the French considered denationalized and a lawful prize. Essentially a set of domestic French regulations, Napoleon's continental system remained legal in territories under French control, in the dominions of its allies, or in consenting neutral countries. The system amounted to "a fantastic blockade in reverse." Its main purpose was not blockade but the ruin of British commerce, as Napoleon himself admitted. "It is by dominating all the coasts of Europe that we shall succeed in bringing Pitt [the Younger, then prime minister] to an honorable peace," he had written in 1800, but "if the seas escape us, there is not a port, not the mouth of a river, that is not within reach of our sword." By denying his adversary access to continental markets, the emperor hoped to destroy British power.
Faced with competing blockades (the British, for their part, desired only to push their own goods onto the Continent, contrary to Napoleon's desire), confronted with ever more restrictive practices on the part of the European belligerents, the neutral United States twisted and turned, without finding a satisfactory resolution of its dilemma. On one occasion President Jefferson told the French minister in Washington that "we have principles from which we shall never depart. Our people have commerce everywhere, and everywhere our neutrality should be respected. On the other hand we do not want war, and all this is very embarrassing." The situation called for action, but action risked war. Under such circumstances, and given the peaceful proclivities of the Jeffersonians, it was tempting to resort to ingenuity; the more so because Napoleon had cunningly remarked in the Milan Decree that its provisions would not be enforced against neutrals who compelled Britain to respect their flag. The president sponsored a series of legislative enactments, including the embargo of 1807–1809, which, through unfortunate timing, coincided with the apogee of Napoleonic power. The baleful effects of the embargo helped convince some Federalists in Boston and elsewhere that the president was in league with the emperor. Nothing could be further from the truth. The European situation remained beyond the influence of American stratagems, no matter how ingenious, as Jefferson and his successor James Madison learned.
The British openly violated their own blockade of Europe by a system of licenses encouraging neutrals to carry both British and colonial goods through the French self-blockade, albeit after those goods had passed through British hands at a profit. In 1807 some 1,600 such licenses were granted; and by 1810 the number had reached 18,000. By that time Russia had deserted the continental system, opening the Baltic to neutral and British trade.
The result of all these twists and turns, taken under the name of blockade, was an extension of the war that soon involved the United States. Unable to control the periphery of his system, in Spain, Portugal, and Russia, Napoleon took his Grande Armée to Moscow and disaster. The object of that campaign, of course, was to force the Russians back into the system and to reconstruct his continental blockade. Shortly before this great effort commenced, the United States, with its sixteen assorted ships of war, entered the conflict against England. Soon the British blockaded ports in the American South and West, although they carefully refrained from blockading New England, where there was much disaffection with Mr. Madison's war. Merchants who carried foodstuffs for British troops received passes through British squadrons. For two months after the declaration of war, the British consul in Boston licensed cargoes.
The wars of 1793–1815 clearly demonstrated the irreconcilable difference between belligerents and neutrals over use of the flexible doctrines of blockade. For the belligerents, especially the naval powers, blockade was a weapon that, if used imaginatively, could do much to bring the enemy to its knees; for the neutrals, blockade constituted a danger to trade and a means of involvement in the war. To the extent that a neutral acquiesced in "unlawful" definitions, that nation decreased its impartiality by actions that gave sustenance to one side while denying it to the other. Conversely, a too vigorous assertion of neutral rights might involve the nation in war. Still, the imprecisions inherent in formulations satisfactory to all, hence to none, provided loopholes that required no great legal legerdemain to stretch meanings to fit the exigencies of a particular war. By selecting from an assortment of precedents and practices, a belligerent could easily define the rules of blockade so as to make neutral commerce a victim of the drive for victory.
When the British sought to close the Continent to neutral trade or to control that trade in their own interest by whatever arbitrary or quasilegal means they might devise, their higher objective, the destruction of Napoleon's warmaking capacity, took precedence over abstract, poorly defined, and largely unrecognized neutral rights and theoretical definitions of how the Royal Navy might or might not use one of its most powerful weapons. In like manner, when Napoleon's continental system came into conflict with American views of proper conduct, the emperor proved no less ingenious or heavy-handed in bending practice to fit his military or economic objectives. Between the infringements of the British and French, Americans had little to choose. Caught between implacable forces in the war that raged over Europe for nearly a generation, Americans struggled to define and defend principles for which the world, at that dangerous time, could find no use. When war threatened the safety of the state, right gave way to might. In its life-or-death struggle for national existence, Britain could not countenance interpretations of blockade that interfered with the pursuit of victory. Failure to understand that fact of international life did much to embroil the United States in a war it did not really want.