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The U.S. Understanding of the Laws of Naval Warfare in the 18th and 19th Centuries

In the 18th and 19th centuries, the United States based its rules of engagement at sea on the American understanding of the law of nations. The law of nations, or international law, as it relates to the sea had its roots in the writings of several 16th- and 17th-century theorists, beginning with the Dutch writer Hugo Grotius, who rejected the claim of the Spanish to dominion over the seas. Grotius argued in favor of mare librum, or freedom of the seas, by which he meant that, by natural law, the oceans are common to all and should be open to all to use without hindrance. Broadly speaking, nations had the right to exercise sovereignty only over contiguous waters that lay within reach of their shore batteries.

1. Neutral Versus Belligerent Rights

            Acceptance of the principle of freedom of the seas left much room for disagreement over the rights of neutrals and the rights of belligerents. In fact, there was no single body of international law accepted by all nations, or even all European nations. Rather, what constituted international law was a continual negotiation between rules that favored belligerent rights and those that favored neutral rights. Whichever rules a nation followed applied equally to its navy and to its privateers.

            It was generally agreed that belligerents had the right to capture merchant ships belonging to subjects of the enemy country, and in order for the commanders of belligerent nations’ warships to exercise this right, they had the right to identify the national identity of merchant ships encountered at sea by boarding them and examining their papers. Resistance to boarding, attempted flight, concealment of papers, dual sets of papers, forged papers, and the like could justify seizure of a merchant ship in time of war. Any merchantman captured by a belligerent warship would have to be sent into a port and given a trial using admiralty law procedures to determine whether the capture was a good prize. Military necessity might justify scuttling or burning a seized merchantman, but in that case, an admiralty trial would still be necessary. If the prize court were to determine that the seizure or destruction was unjustified, the captors would have to return the property to its owners, when possible, and pay damages.

            It was also generally agreed that a merchantman lost its neutral character by giving or attempting to give material aid to one of the belligerent powers. Thus, evidence that the merchantman was carrying contraband goods to the enemy would justify a capture, as would an attempt to carry any goods into a port blockaded by a belligerent. But a neutral engaged in innocent trade with an enemy through a port that was not under blockade should be allowed to go about its business. Even certain classes of vessels belonging to enemy subjects were exempt from molestation: coastal fishing vessels and vessels involved in humanitarian missions, such as scientific exploration.

            Within these general parameters, disagreements over details were legion. What constituted contraband? What constituted a legitimate blockade? Were neutral goods carried in an enemy’s merchantman subject to capture? Were enemy goods carried in a neutral merchantman exempt from capture? Did the transporting of enemy diplomats compromise a vessel’s neutrality? Did the enemy character of a vessel depend on the citizenship or the residence of the owner? Should a neutral merchantman under naval convoy be exempt from visit and search?

            From the Declaration of Independence in 1776 to the U.S. declaration of war on Germany in 1917, the contest between neutral and belligerent maritime rights was a major focus of U.S. foreign policy. The U.S. government generally pressed for adoption of international rules that favored neutral traders. The United States sought recognition of a rule that would treat enemy goods in a neutral merchantman as neutral property and thus, with the exception of contraband, make such goods exempt from confiscation; this was known as the principle of “free ships make free goods” in which the flag covers the goods. The U.S. government held that a neutral’s goods in an enemy vessel were also to be treated as neutral. The United States favored limiting the nature of goods to be considered contraband, supplies intended for an enemy that a belligerent could legitimately confiscate from a neutral vessel, even at times promoting elimination of the notion of contraband entirely. Eliminating the right of belligerents to seize contraband would, in turn, limit the practice of warships visiting and searching neutral vessels to those merchantmen attempting to enter a blockaded port, thus resolving another contentious issue in American foreign relations. The United States asserted that neutrals had the right to trade non-contraband goods between a belligerent’s ports. The U.S. government insisted that a blockade was legal only if it was publicly advertised as well as being effective enough to present a merchantman attempting to enter a port realistic danger of capture. Moreover, the United States promoted the immunity of private property at sea.1         

2. Rules of Engagement

            The rules for engaging in combat with an armed warship or fleet of the enemy were fairly simple. A commander of a belligerent warship or fleet would do his best to identify the nationality of another warship or fleet encountered at sea. This process was complicated by the practice of flying false flags in a deliberate effort to deceive. The only rule about flying false flags was that a warship was obliged to hoist its true colors before firing the first shot. There were incidents of friendly ships firing into each other because of errors into which the use of false flags led them. If the commanding officer wished to request a pause in the fighting for the purpose of a parley, he could raise a white flag. A white flag was not a signal of surrender and the opposing commander was not obliged to honor it. A commander would signal surrender by lowering the ship’s colors. It was a violation of international standards to continue firing after lowering the colors in surrender as it was to continue firing into an enemy ship that had clearly surrendered. A ship captured in the service of the enemy during battle immediately became the captor’s property and was not subject to an admiralty prize trial.

            The standard procedure when a belligerent warship encountered a merchant vessel at sea was for the belligerent to hail the merchantman, indicating that it should haul up to be boarded. If the merchantman ignored the hail, the belligerent would fire a warning shot across its bow. If the merchantman still did not stop or if it fled, the belligerent could take actions required to force the merchantman to stop and be boarded, including firing into the ship. Simply hailing the merchantman, the belligerent could display false colors, but before firing into the merchantman, the belligerent was obliged first to hoist its true colors. Firing into a merchantman without warning or before displaying one’s true colors was considered a violation of international norms, and sinking a merchantman without first removing crew and passengers would be condemned as unjustifiable homicide.

3. United States Neutrality and World War I

            Representatives of the world’s leading naval powers met at The Hague in 1907 to agree on rules regulating naval warfare and at the London Naval Conference of 1908–1909 to settle differences among them in interpretation of prize law. The London conference focused exclusively on matters affecting relations between neutrals and belligerents. Even though the conference produced a compromise agreement that balanced the rights of neutrals and belligerents, none of the powers would formally ratify the Declaration of London.

            With the onset of World War I, both the United Kingdom and Germany violated American notions of international law. The Royal Navy interdicted neutral trade with Germany well beyond the limits that would have been consistent with American principles, and German U-boats sank merchant ships supplying the United Kingdom without giving the customary warning.

            Early in the war, while the United States was still neutral, the administration of Woodrow Wilson demanded that the British not merely adopt the rules governing commercial warfare agreed to in the Declaration of London, but also institute the principle of immunity from maritime capture of private property, leaving all merchantmen, enemy as well as neutral, unmolested. The British responded by stating that they would mostly follow the Declaration of London’s rules, but with a few modifications. Most important of these modifications (despite the fact that the Declaration of London, in accordance with traditional international law, defined foodstuffs as “conditional contraband,” which could only be interdicted if its intended destination could be proved to be enemy armed forces) was the British decision to treat all foodstuffs destined for Germany as contraband, even if heading for a neutral port—in contravention to the provision that the notion of “continuous voyage” could not be applied to conditional contraband. Soon after, the British added a long list of raw materials to its definition of contraband and began seizing or detaining neutral ships, including American, headed for ports in Europe, including neutral ones, through which those supplies might reach Germany. All of this was in clear violation of established international law. Nevertheless, the United States made no formal protest to the British policy and in practice acquiesced in this violation of its rights as a neutral, and Germany justifiably complained that the United States had joined in Britain’s campaign to starve Germany, thereby compromising American neutrality.

            This all occurred in the first months of the war, before German submarines began sinking enemy merchant ships in the waters surrounding the United Kingdom without giving the customary warning and without allowing the crew and passengers to abandon ship. The German government recognized that this policy went against traditional international law, but argued that German’s vital interests were involved—the same justification the British used to defend its illegal blockade of Germany which the neutral nations of the world had accepted. The Germans realized that submarines were vulnerable to destruction by armed merchantmen or merchantmen capable of sinking submarines by ramming; they conceived that by arming themselves, or acting under orders to ram submarines, merchantmen eliminated any claim they had to advance warning before being sunk. In contrast to its response to British violations of America’s neutral rights, the Wilson administration immediately, formally, and bluntly protested Germany’s unprecedented campaign of commerce destruction by submarines.

4. World War II Produces a Revolution in U.S. Policy on the Rules of Engagement

            America’s World War II contest with Japan put an end to America’s traditional view of freedom of the seas. As historian Joel Ira Holwitt writes, the order to conduct unrestricted submarine warfare against Japan, targeting “all Japanese shipping, from fishing trawlers to freighters to tankers,” issued on 7 December 1941, was “a major and dramatic change to the American attitude toward freedom of the seas.”2 In World War II, the U.S. Navy exercised the type of unrestricted submarine warfare the United States had condemned in Germany in World War I.

            Today, U.S. naval officers will find instruction on the rights of belligerents and the rights of neutrals in The Commander’s Handbook on the Law of Naval Operations (NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7A, Edition July 2007 [Washington, D.C.:  2007]), which is an update of the Law of Naval Warfare, promulgated by the Navy Department in 1955. The 1955 volume in the U.S. Naval War College’s International Law Studies series, The Law of War and Neutrality at Sea, by Robert W. Tucker, is a commentary on the Law of Naval Warfare. Tucker’s main concern in writing the 1955 Naval War College volume was to explore the relationship between modern belligerent practices and the traditional law of naval warfare. He concludes that the modern practice of sinking enemy merchantmen without warning is in accord with the principles behind the traditional law. The standard international law texts, Grotius, Vattel, and others, were written during an era in which merchant vessels did not pose a serious threat to warships, and therefore the accepted practice was to require a belligerent warship, before firing on an enemy merchantman, to identify itself and give the enemy merchant ship a chance to surrender. At the beginning of the 20th century, the vulnerability of submarines to sinking by armed merchantmen altered this calculus: It is unreasonable, says Tucker, to expect warships to expose themselves to the probability of their own destruction. Tucker argues that it was both the arming of merchantmen against vulnerable submarines and changes in the way nations at war employed merchantmen, making them in effect part of the military forces that rendered them, like enemy warships, subject to unannounced sinking in accordance with traditional international law. He writes, “It is difficult to avoid the conclusion that the immunity granted merchant vessels by the traditional law can be observed only under the conditions that merchant vessels do not present—in terms of their armament—a serious threat to enemy warships and that they are in no way integrated into the military effort of a belligerent. If either, or both, of these conditions do not obtain, and they were not satisfied even in World War I, warships—whether submarines or surface vessels—cannot be expected to refrain from attacking enemy merchant vessels….These novel circumstances are—from the present point of view—neither the submarine (and aircraft) nor the central importance of what has come to be known as economic warfare, but rather the insistence of belligerents upon the resort to measures which have as their direct consequence the integration of merchant shipping into the military effort at sea.”3


— Dr. Michael J. Crawford, Senior Historian,  NHHC, September 2017


1 On the U.S. policy favoring international adoption of the principle of immunity from maritime capture of private property, see Michael J. Crawford, “Taking the Moral High Ground: The United States, Privateering, and the Immunity of Private Property at Sea,” International Journal of Naval History 12 (Jan. 2015),

2 Joel Ira Holwitt, “Execute against Japan: The U.S. Decision to Conduct Unrestricted Submarine Warfare (College Station: Texas A & M University Press, 2009), 1–3. See also Janet M. Manson, Diplomatic Ramifications of Unrestricted Submarine Warfare, 1939–1941, Contributions in Military Studies, No. 104 (New York: Greenwood Press, 1990), 1. See both of these works for the story of how and why the United States made this momentous decision.

3 Robert W. Tucker, The Law of War and Neutrality at Sea, Naval War College International Law Studies 50 (Washington: Government Printing Office, 1955): 55–73, quotation on 62–63.


Published: Tue Sep 10 12:42:19 EDT 2019