Specifically, what was the first time somebody attempted to bring together representatives of all known nation states (where reasonably practicable in terms of logistics and development), without prejudice, for a single cause - be that political, sporting, trade or leisure?
For example, the Hanseatic League was an early attempt to direct trade among multiple states, so could be considered international, but does not meet the terms of this question because it did not attempt to include all known states.
From whose perspective? I would venture that there has not yet been an organization which met those criteria.
I wrote a paper on this back in grad school. I think it is pertinent to at least part of your question. Forgive the length and the inconsistent citations - this work never advanced past the draft stage with my advisor:
Roads not taken: abortive efforts at state-to-state accountability
Western adjudicatory bodies did not arise spontaneously in the eighteenth century from the international ether: there is a rich history of proposed supranational bodies that dates back at least as far as the middle of the 15th century, when the Bohemian King George of Podebrady, “proposed the organization of the Christian rulers of Europe into a federation or league with considerable powers." (Araujo and Lucal.) This organization would’ve recognized each member state as equal regardless of economic or military power, and its multinational character was remarkable for the time. George’s object was the defense of Christendom from the looming threat of the Ottomans (indeed, his proposal was prompted in large part by the loss of Constantinople in 1453): “George of Podebrady, King of Bohemia['s] aim was a 'Christian' peace... he designed the structure of Christian unity, he almost drafted its constitution." (Kende.) Yet George’s scheme failed in large part to conflicts with the regnant papacy of the time, in spite of years of negotiation among multiple European states. His plan laid the groundwork for the proclivity among progressive European thinkers to suggest a supranational federal body to adjudicate disputes and enforce international law, however.
In 1603, King Henry IV proposed a “Grand Design,” partitioning Europe into fifteen independent powers, each possessing a seat on a General Council of the continent modeled directly on the Amphictyonic League of the Greeks. Henry suggested four representatives from each power would sit in a European senate, each representative holding a term of three years. (Darby, pp. 14-18) This General Council would adjudicate disputes between members and it’s “decisions should be like irrevocable and unchangeable decrees…” (Darby, p.18) The Council would even possess armed forces, paid for by member states, and was invested with the power to punish member states by Council decision. (Ibid.) Henry’s plan was doomed to fail for several reasons: firstly, because Henry subscribed to the thoroughly fanciful notion that the nations of Europe would happily submit to a process of political recombination (a notion assuredly odious to many at that time). Secondly, Henry’s plan lacked specifics for supporting the design, electing to leave up to the General Council’s eventual members all particulars regarding how voting would be conducted, how decrees would be promulgated and enforced, how taxation for the organization would proceed, etc. Finally, and most critically, Henry’s plan was chiefly a cynical effort designed to disenfranchise his Austrian rivals: “The political part of the Plan… was to despoil the House of Austria of all its possessions in Germany, Italy and the Netherlands- in a word, to confine it to the kingdom of Spain… [and its possessions in the Americas.]” (Ibid.) The Quaker statesman William Penn proposed a like scheme in 1693 for a European federal Diet. Penn’s plan was considerably more detailed than the late Henry’s: it specified that the Diet meet at least annually, that all disputes between sovereigns be brought before it, that the Diet would have the power to “seek their remedy by arms” should a member state renege on its attendant obligations, and that the representation be proportionate to population and taxation be apportioned to each member state. Penn proposed a voting practice by ballot, with no measures passing except by a three-quarters vote or a majority of seven or more votes. “It is extremely necessary that every Sovereignty should be represented at the Diet under great penalties,” Darby concludes, stressing Penn’s commitment to an equality of nations. (Ibid., 20-24) Penn’s proposal had no effect on European diplomacy.
In 1714 the French clergyman and radical Charles-Irénée Castel de Saint-Pierre published an updated version of Henry’s Design for a European federal society. Saint-Pierre’s views were thoroughly forward thinking. Hont writes that “Saint-Pierre [suggested a] newly-formed superstate... a confederacy, a 'European Union,' mixing modern sovereignty with the federal system that worked rather well in the large but fragmented German territories... Saint-Pierre wanted to put an end to the jealousy of state." (Hont, p. 27) Saint-Pierre’s design contained provisions for the establishment of a European assembly, rules prohibiting armed conflict between member states, and intricate instructions on how to assign senators from the various European member states, as well as thorough by-laws regulating the assembly’s actions too lengthy to describe in detail here. More relevant to this study, Saint-Pierre’s design declared that “Each Sovereign shall, at his own charge, exterminate his inland robbers and banditti, and the pirates on his coasts, upon pain of making reparation…” (Darby, p. 28) One detects an affinity between this declaration and de Vattel’s later contention that “[pirates] attack and injure all nations, by trampling underfoot the foundations of their common safety.” (de Vattel) Further, Saint-Pierre proposes that, following a member state’s engaging in an illegal war, “two hundred of the principal ministers or officers of the enemy… shall be delivered to the Union, and punished with death or imprisonment for life.” (Ibid., p. 34) Finally, Saint-Pierre delineates “temporary Committees, formed expressly to reconcile differences between Sovereign and Sovereign.” (Ibid., p. 46) The design provides for a provisional judgment by an arbitrator, followed by two ratifying votes from the grand European assembly. This prototypical International Court of Arbitration was a serious, if untaken at the time, step forward in international jurisprudence. Saint-Pierre’s unorthodox political philosophy earned him few friends in late 17th and early 18th century France, and he was subsequently expelled from the Académie Française for, among other things, promulgating his plans for a federal republic of Europe.
Sources for further reading
Araujo, Robert J., and John A. Lucal. "A Forerunner for International Organizations: The Holy See and the Community of Christendom: With Special Emphasis on the Medieval Papacy." Journal of Law and Religion 20.2 (2004). Web. 17 Dec. 2011.
Bederman, David J. International Law in Antiquity. N.p.: Cambridge University Press, 2001. Print.
Boczek, Boleslaw A. Historical Dictionary of International Tribunals. N.p.: Scarecrow Press, 1994. Print.
Darby, W. E. International Tribunals. N.p.: International Law Association, 1897. Print.
Hudson, Manley O. International Tribunals: Past And Future. 1944: Carnegie Endowment for International Peace, n.d. Print.
Kende, Istvan. "The History of Peace: Concept and Organizations from the Late Middle Ages to the 1870s." Journal of Peace Research 26.3 Aug. (1989). Web. 17 Dec. 2011.