Diplomacy is the art of conducting the intercourse and adjusting the mutual relations of nations. The term owes its origin to the ancient use of public documents, known as diplomas, from their being written on two leaves or double tablets. From the point of view of the philosophy of law, diplomacy arises out of the necessary interdependence of states. This interdependence being recognised, the rights and duties of political intercourse flow from the rights and duties of recognition; and it is, consequently, from the general doctrines of the recognition of state-existence that the special principles of legation are to be deduced. Recognition implies separate existence, not potential only, but actual, and therefore only states having such separate existence are entitled to express their will through separate diplomatic agents. It is on this ground that the right of legation is denied to colonies, however important or distant from the parent state. A limitation in the rights and duties of legation also arises in the case of semi-barbarous nations, whose municipal law and the judgments of whose courts are not recognised by civilised states. Diplomacy has arisen out of the development of the European powers, and, as a uniform system, it is even now confined chiefly to these powers. Its practical rules are embodied partly in those international customs and usages which constitute what may be called common, and partly in those treaties which may be regarded as statute international law.
The frequent necessity for rapid decision in this department of politics has compelled even those nations who most jealously guard their constitutional rights to intrust at least provisional power of action to individual rulers. Thus in Britain the sovereign, independently of parliament, has technically the power to make peace and declare war. The practical guidance of the relations of Britain with foreign states is committed to the Secretary of State for Foreign Affairs and his department. The power, however, of sending ambassadors to, and receiving ambassadors from foreign nations remains an unalienable privilege of the crown. It was doubted whether an exception had not been made in the case of Rome, by the statutes passed against papal encroachments; but such doubts were removed by 11 and 12 Vict. chap. 108, which authorises the sovereign to enter into diplomatic relations, provided that no person in holy orders in the Church of Rome, or Jesuit, or member of any other religious order, community, or society of that church, bound by monastic or religious vows, shall be received as ambassador in London.
The existing diplomatic hierarchy, as fixed by the annex to the Treaty of Vienna in 1815, falls into three ranks. (1) Ambassadors, legates, or nuncios, who alone have the representative character; (2) Envoys extraordinary, or ministers plenipotentiary, accredited to sovereigns; (3) Chargés d'Affaires, who are entitled to transact business only with the Minister for Foreign Affairs. Every diplomatic agent must be furnished with a letter of credence stating the general object of his mission, and requesting that full faith and credit be given to what he shall say on behalf of his court. From the moment that a public minister enters the territory of the state to which he is sent, until he leaves the country, he is entitled to an entire exemption from the local jurisdiction, both civil and criminal. An English ambassador, with his family and suite, whilst abroad in the public service, is held to be domiciled in England; his house is on English ground, and he carries the municipal laws of his own state along with him. Debts incurred in his public capacity must be sued for in England, and, in the event of his transgressing the laws of the foreign nation to which he is accredited, he can be dealt with only diplomatically—i.e. England must be called upon to punish him. An ambassador, as representing a sovereign power, ranks in the court to which he is accredited immediately after the princes of the blood-royal.
The international law of Europe has attributed to certain states what are called royal honours, which entitle the states by whom they are possessed to precedence over all others who do not enjoy the same rank, along with the exclusive right of sending to other states diplomatic agents of the first rank. Such royal honours are enjoyed by the empires and kingdoms of Europe, and amongst Catholic states by the pope; and the same right extends to the United States of America. Where the rank of different states is equal or undetermined, different expedients have been resorted to for the purpose of avoiding a contest, and at the same time securing the respective rights and pretensions of the parties. This subject was left by the Congress of Vienna on the ancient footing of custom. The most important of these expedients is what is called the alternat, by which the rank and place of the various powers are changed from time to time, either in a certain regular order, or in one determined by lot. Thus, in drawing up public treaties and conventions, it is the usage of the powers to alternate, both in the preamble and the signatures, in such a manner that the name of each state stands first in the copy intended to be delivered to it, whilst the others are arranged in the order determined by lot.
Colloquially the term is frequently applied to conduct which, if not altogether fraudulent, is characterised by a certain degree of cunning and subtlety. This use of the word probably arose from the popular impression that, in conducting the affairs of nations, there is in use a code of morality which would be condemned if practised by individuals in their intercourse with each other. Nor, judging from the old literature of legation, can it be said that this popular conception of the ambassador and his functions was altogether unfounded. Of late years, however, the general progress of international morality, and more advanced conceptions of international relations, have considerably modified the older view of the functions of diplomacy and diplomats. Speech in the mouth of a diplomatist is no longer an instrument for the concealment of thought; an ambassador is no more a functionary commissioned 'to lie abroad for the advantage of his country,' but one selected to tell the truth on her behalf. In conducting honest negotiation with a view to preserve peace among the nations, so long as it is consistent with the honour of his country; in seeing that the rules of justice are observed among men; in preserving the weaker states against the more powerful; and in the less imposing, though not less difficult duty of watching over the equitable administration of the minor rules of international law, there lies before the modern diplomatist a sphere of action as honourable as it is arduous.
General convenience early suggested the use of one language in diplomatic intercourse. For many centuries Latin was the ordinary medium of political correspondence, but the preponderance of Spain towards the end of the 15th century contributed to the general diffusion of the Castilian tongue for this purpose. This again has been superseded by the language of France, which, since the age of Louis XIV., has become the almost universal diplomatic idiom of the civilised world.—The phrase Corps Diplomatique is usual for the whole body of ministers who are present at any court as representatives of foreign countries.