From the end of the Civil War to the opening of World War I in 1914, the United States did not concern itself greatly with the freedom of the seas. It was a neutral in three wars during the period (Franco-Prussian, Boer, Russo-Japanese), but none of them presented any serious problems on the seas. The United States was a belligerent once during this period (against Spain), but that conflict was too brief to raise any serious maritime issues. There was, however, one significant development in American policy toward neutral commerce during the Spanish-American War—the division of the contraband list into absolute and conditional contraband. The former included articles primarily and ordinarily used for military purposes and destined for an enemy country; the latter included articles that might be used for purposes of war or peace, according to circumstances, and would be subject to seizure only if actually and specifically consigned to the military or naval forces of an enemy. In the latter category, foodstuffs and coal were the most important items. This division became a permanent feature of American policy when it was incorporated into the United States Naval War Code, adopted in June 1900.
If there was one preoccupation of American diplomacy concerning neutral and belligerent rights and duties, it was the effort to secure international acceptance of the principle of the immunity of private property at sea. The adoption of such a broad principle, long sought by American diplomats, would have applied to all private property, both neutral and belligerent, replacing the more specific provisions covering neutrals, such as "free ships make free goods." It was the subject of negotiation with the North German Confederation in 1870, and was incorporated into a treaty of amity and commerce with Italy in 1871. In December 1898, President William McKinley asked Congress for authority "to correspond with the governments of the principal maritime powers with a view of incorporating into the permanent law of civilized nations the principle of the exemption of all private property at sea, not contraband of war, from capture or destruction by belligerent powers." Five years later his successor, Theodore Roosevelt, reiterated the plea. In the instructions prepared for the U.S. delegation to the First Hague Conference in 1899, the chief item was on immunity of private property at sea; the instructions for the delegates to the Second Hague Conference in 1907 included a congressional resolution of 1904 supporting the same principle. All these efforts proved in vain, however. The United States did not succeed in gaining international acceptance of the doctrine.
The United States did, nonetheless, have the satisfaction of seeing many of its other principles adopted at an international congress that met in London during the winter of 1908–1909, convened at the call of Great Britain. The ten maritime powers represented (Germany, England, Austria-Hungary, the United States, Spain, France, Italy, Japan, the Netherlands, Russia) agreed on a code of prize law that would be administered by an international prize court hearing appeals from national prize courts set up by belligerents. In seventy-one articles contained in ten chapters, precise and detailed rules were established governing blockade, contraband, nonneutral service, treatment of prizes, determination of a vessel's character, convoy, transfers to a neutral flag, and visit and search of vessels. Taken together, the rules that made up the Declaration of London were favorable to neutrals, which may account for the fact that the British House of Lords refused to ratify them (after the House of Commons had given its approval). Thus, they were not binding on any of the other signatories.