With the close of the Napoleonic wars in 1815, attendant upon Waterloo and the emperor's banishment to St. Helena, there followed almost a century when, with the possible exception of the American Civil War, no major conflict involving neutral rights took place. The important wars of the nineteenth century, from 1815 to 1914, were either civil conflicts such as the Taiping rebellion in China (1850–1864) and the American Civil War, or land wars of relatively short duration such as the Franco-Prussian War (1870–1871). Commentators on the law of war therefore had ample opportunity to refine their concepts and to sharpen their definitions. Statesmen of the time, especially American leaders, stressed the need for a new, more reasonable international order. As secretary of state, Madison had set the lines of this litany when he denied the legality of a British blockade of the entire islands of Martinique and Guadeloupe. In 1805 he told the British chargé d'affaires in Washington that international law required the presence of sufficient force to render "access to the prohibited place manifestly difficult and dangerous." In defense of this doctrine—and for the sake of American exports of food and naval stores—he added: "It can never be admitted that the trade of a neutral nation in articles not contraband, can be legally obstructed to any place, not actually blockaded." In 1824, Secretary of State John Quincy Adams ventured a new definition of a legal blockade, one that required "ships stationary or sufficiently near" the place prohibited, so that there was "evident danger" in attempting an entry. Then, during the Mexican War (1846–1848), the United States again affirmed its opposition to nominal blockades by telling the neutral British that according to American rules, "no Mexican port was considered blockaded unless a force was stationed sufficiently near to make trade with that port dangerous."
The United States refused to adopt the 1856 Declaration of Paris by which the major European powers at the end of the Crimean War attempted to promulgate a new code of neutral rights. That set of rules included a revised definition of blockade: "Blockades, in order to be binding, must be effective—that is to say, maintained by a force sufficient really to prevent access to the coast of the enemy." That article coincided with traditional American views. Other portions of the declaration did not measure up to Washington's expectations of what a proper code of conduct should be. During the war, with the fighting mainly on land and hardly touching neutral commerce, the maritime powers, France and Britain (then allied against Russia), realized that privateers, that is, legalized private ships of war, might prove attractive to the Russians in some future war. Having renounced use of such vessels during the war, the victorious allies sought in peace a formal international prohibition against their use. Part of the price for such an abolition was adoption of a more liberal view of neutral rights, and the powers of Europe, including Britain, subscribed to the rules set out at Paris. Hence, the ideal statement about blockade. But the American government, like the Russian, found fault with the new code. With its small navy, the United States might find future utility in use of private vessels of war and was therefore reluctant to surrender their use. Until belligerents were willing to afford a total immunity to all private property at sea, the Americans did not want to abolish privateering. They sought, rather, to trade off American acceptance of the article abolishing privateers for European recognition of the principle of immunity for private property at sea during wartime.
Failure to sign the Declaration of Paris did not enhance America's status as a champion of expanded neutral rights or as a proponent of the need for clear limits to blockades. Nor was the American position advanced by the circumstances of the Civil War, a conflict that became so heated—the need to contain the rebellious South being so pressing—that Washington officials proved willing to abridge the national record on the rights of neutrals, particularly in the use of blockade theory and practice, if only such abridgment would bring victory. Indeed, some observers and later historians have argued that the United States had been a champion of neutral rights when it had a small navy and little military power, but when it marshaled the most effective army and largest navy in the world, it jettisoned the principles of an earlier generation in favor of a more expedient approach.
In retrospect, the Civil War seems to have been so large an anomaly in American national life that no easy judgment can be made on whether President Abraham Lincoln and his aides forsook the principles of the Founders to save the Union. For the president and his secretary of state, William H. Seward, the fundamental international problem during the war was to preserve the neutrality and if possible the goodwill of Britain. Blockade measures against the South therefore had to be arranged so as to put maximum pressure on the Confederacy without provoking British reprisals. To be sure, other European neutrals occasionally encountered difficulties—Spain, for example, because of ownership of Cuba, from whence blockade-runners sometimes passed, and Denmark because of the proximity of the Virgin Islands to the Confederate coast—but their involvement never reached crisis proportions. The Mexican government frequently complained about actions by Union captains off Matamoros, contrary to the Treaty of Guadalupe Hidalgo (1848), which forbade any blockade of the Rio Grande. But the British response to problems generated by the war always concerned Union leaders most, for they realized that Britain's international position, its merchant fleet and naval strength, gave Her Majesty's government a vital interest in transatlantic affairs. For example, when the British almost from the outset of the war failed to push any blockade cases with the American government, that forbearance provided Lincoln's administration with a helpful leeway in manhandling aid for the South coming in by sea, or any effort by Southerners to ship cotton abroad in order to import the arms and supplies needed to prosecute the war. Britain's lack of militancy on issues concerning blockade became so marked by the second year of the war that the federal government in Washington enjoyed virtual carte blanche in its measures to seal off the South from supplies.
At the outbreak of war the Lincoln administration made a slip, when on 19 April 1861 it proclaimed a blockade of Southern ports from South Carolina to Texas and then eight days later of the ports of North Carolina and Virginia. The president should have declared the ports closed. Proclamation of a blockade was a presumption that the South enjoyed belligerent status and might merit international recognition as an independent nation. Officials in London felt that, in any event, they could not look upon 5 million people as pirates or as engaged in unlawful combination, and on 13 May they issued a neutrality proclamation, which included a warning to British subjects against the violation of any blockade established by either belligerent. Months later, in July, President Lincoln tried to amend the legal faux pas by saying that the blockade was "in pursuance of the law of nations" against a domestic insurrection: "A proclamation was issued for closing the ports of the insurrectionary districts by proceedings in the nature of blockade." These changes in legal terminology did not result in European withdrawal of recognition of Confederate belligerent status.
Later, debate would focus on the effectiveness of the Union blockade or on the "proceedings in the nature of blockade." Writers have contended that Union efforts were effective, but one twentieth-century southern historian argued that the blockade was a sieve. He calculated that blockade-runners made 8,000 trips to the South. He further points out that, in the early stages, the Union did not have sufficient ships to give even a semblance of effectiveness to its declaration. The porous nature of the blockade invited attempts to run into Southern ports with profitable cargo. Many of the adventurers who tried their hand at the business were "retired" British naval officers and other subjects of Her Majesty, the Queen. So many Britons took part in blockade-running that Lord John Russell, the foreign secretary, offhandedly quipped that his countrymen would, "if money were to be made by it, send supplies even to hell at the risk of burning their sails." Throughout the war, profits remained high; a return of 1,000 percent upon investment was not uncommon. Even in 1864, a captain who ordinarily made $150 per month might earn $5,000. A popular toast celebrated the blockade-runners' thankfulness to everyone: "The Confederates that produce the cotton; the Yankees that maintain the blockade and keep up the price of cotton; the Britishers that buy the cotton and pay the high price for it. Here's to all three, and a long continuance of the war, and success to blockade-runners." Still, the blockade was effective enough for the British government, despite considerable pressure against the move, to recognize its existence. Lord Russell, who can hardly be described as pro-North in outlook, eventually concluded that the Union blockade had to be considered "generally effective against foreign trade." His minister in Washington, Lord Lyons, regarded it as more than a mere paper blockade, noting that if it were "as ineffective as Mr. Jefferson Davis says … he would not be so very anxious to get rid of it." From reports of the commander of their North American station, the British carefully monitored the performance of Union blockading squadrons, and not until early 1862 did they formally accept the blockade. In February of that year, Russell told Lyons that there were enough Union vessels on blockade duty to prevent access to Southern ports or "to create an evident danger" to ships seeking to enter them.
Another Southern wartime hope—that the need for cotton would force European powers to press Lincoln's government to relax its blockade— also proved illusory. As it turned out, King Cotton proved a weak champion and an inept diplomat. By chance, the crop of 1860, one of the largest on record, had been shipped to Europe before the war started, and by the time a shortage developed, in the winter of 1862–1863, the South's military position was too precarious to warrant European intervention in American affairs.
One additional facet of the Civil War blockade deserves mention. Four captures of ships made during the first months of the war raised questions about the right of the federal government under international law to establish a blockade of its own ports during an insurrection, and of the right of the president to do so in the absence of a congressional declaration of war. Attorney General Edward Bates was advised to delay the cases until the president could appoint more politically reliable justices to the Supreme Court. After three Lincoln appointees joined the court, the government's position on the utility of the blockade was barely upheld in 1863, by a vote of five to four. The five cooperative justices made up in ardor what they lacked in support from their less certain brethren; and the dissenters no doubt blanched to hear that they had taken the "wrong" side of an issue involving "the greatest civil war known in the history of the human race," and that their negative arguments had threatened to "cripple the arm of the government and paralyze its power by subtle definitions and ingenious sophisms."
In one of the prize cases, that of the Springbok, the court ruled that any cargo ultimately intended for a blockaded port could be captured whenever it left the territorial waters of its port of origin. This rule applied, the court said, "even if the cargo was to be transshipped at an intermediate port, and the vessel in which it was found when captured was not the one which was to carry it to a blockaded port." In this case the court assigned a penalty for a breach of blockade "to a guilty cargo in an innocent ship." The court, in effect, ruled that the cargo was on a continuous voyage from its port of origin to a blockaded port. Acceptance of this definition increased the power of the Union navy in intercepting supplies en route to the South. Such rulings went far toward making a blockade of Confederate ports almost unnecessary by substituting what amounted to a paper blockade of neutral ports in the Caribbean and Mexico. The case of the Peterhoff raised a question of the shipment of contraband overland, from Matamoros, Mexico, across the Rio Grande to Brownsville, Texas. The Union navy found itself with a perplexing problem in Matamoros, which before the war had had scarcely half a dozen visiting vessels a year. This hardly vibrant entrepôt welcomed 200 ships by 1864. Union captains hesitated to move against a neutral port, but they took ships en route to it, the Peterhoff being one of their more famous captures. After the war the case of the Peterhoff came before the Supreme Court; Chief Justice Salmon P. Chase roundly affirmed America's traditional record of respecting neutral ports and internationalized rivers such as the Rio Grande; he also asserted that the nation did not favor paper blockades. Release of the ship seemed a reasonable price for so many reassuring affirmations.
In another ploy to increase the efficacy of the blockade, Union officials even refused clearances to suspicious cargoes from their own ports or required the posting of heavy bonds to assure that such cargoes were intended for peaceful purposes. (During the war, suspiciously large amounts of clean-burning anthracite coal, a key component of successful blockade-running, were being shipped to British ports in Canada and the Caribbean.) The British chargé d'affaires complained about these export restrictions, remarking that the congressional enactment that sanctioned them was "a cheap and easy substitute for an effectual blockade." These practices so irritated British merchants that in 1864 Lord Lyons threatened that Her Majesty's government might have to reconsider its recognition of the legality of the Union blockade. Secretary of State Seward knew that, so late in the war, such an action could not serve British interests, so he ignored the minister's protest. For the remainder of the war the Union continued to use all the legal and economic weapons that it possessed to defeat the South.
During the half-century from the close of the Civil War to the opening of World War I, there were only one or two refinements in the concept of blockade. As noted, the Franco-Prussian War provided no opportunities for the expansion of old definitions or the creation of new ones. The Boer War and the Spanish-American War were local conflicts—the one a civil war, the other a splendid little affair. In the course of operations before Manila, prior to the capture of that city in 1898, Admiral George Dewey came to dislike the pretensions of a German admiral who happened to be in the harbor, and there is some evidence that Dewey told his German opposite that he wanted to "damn the Dutch." But that was hardly a refinement of blockade; nor did anything of a novel nature accompany use of blockades in the Caribbean, although the closing off of Santiago de Cuba was one of the last pre-submarine, close-in blockades, while the one at Havana, which began on 22 April 1898, had a closer connection with strategic considerations than with economic ones. The Americans believed that they could not capture Havana Harbor without risking heavy losses.
More important developments took place in a contretemps before the ports of Venezuela in 1902, when Britain, Germany, and Italy instituted a blockade to collect the debts owed by Venezuela to European creditors. The United States served notice that there was no right to interfere with ships of third parties, that is, American ships; whereupon the blockading powers announced that their blockade "created ipso facto a state of war" and gave themselves belligerent rights. When the issue went to the Permanent Court of Arbitration, the tribunal carefully skirted questions about the legality of the blockade, but in adjudicating the claims of the creditors decided in favor of the blockading powers. Shortly thereafter the nations of the world made another illustrious, if inconsequential, pronouncement about blockade. The second Hague Peace Conference (1907) had sought to establish an International Prize Court, but the delegates could not agree on the rules of prize law. To fill this gap a conference met in December 1908 in London, and after two months the delegates signed the London Declaration. Its provisions on blockade demonstrated once again the great difficulty in arriving at a satisfactory definition; indeed, these men of the twentieth century did little better than their nineteenth-century counterparts at Paris in 1856. Unable to agree upon a definition that would be acceptable to all, and recognizing the need for a "certain imprecision," the delegates reaffirmed the illegality of paper blockades and said that for a blockade to be binding it had to be maintained by an "adequate" naval force. No government ratified the London Declaration, even though during World War I the United States pressed the British to accept its principles. The resultant refusal highlighted a basic ambiguity of international life. Despite the best intentions, a power at war will retain loopholes in commitments to other nations so as to permit maximum use of offensive and defensive weapons.