DIPLOMATIC AGENT. - Organ of political relations between States, charged with representing the sending State, temporarily or permanently, in the State where he exercises his functions. The use of occasional and transitory embassies goes back to the remotest antiquity. It is, on the other hand, only in the second half of the 17th century that the use of exchanging permanent diplomatic representatives became generalized among States (the first permanent representation is the one established by the Republic of Venice at Genoa in 1455). A use that has assumed such a character of generality and regularity that people speak of the right of active and passive legation as an attribute of international personality, and it is debated whether this "right" also belongs to entities other than States and the Holy See. It must be excluded, however, that the exchange of diplomatic agents between two States constitutes an object of right and obligation. Diplomatic agents are distinguished into ordinary and extraordinary. This distinction places on the one hand envoys of a permanent character, who have the task of protecting the interests of the sending State in the State where they are located, observing and reporting to their government, acting as intermediaries for communications, watching over the observance of the commitments assumed toward their government by the foreign government, conducting negotiations, protecting resident nationals, etc.; and on the other hand envoys without a permanent character, charged with special missions, such as the stipulation of a treaty, the presentation of public apologies, an official proposal of marriage, etc.; or with the representation of the State at a congress, at a conference (delegates) or at an official ceremony. According to the regulation adopted at the Congress of Vienna (protocol of the session of 9 March 1815 and additional protocol of Aix-la-Chapelle of 21 Nov. 1818), intended to put an end to questions of precedence that often gave rise to controversy, diplomatic agents are classified into four categories in order of importance: 1) ambassadors, with whom legates and pontifical nuncios are placed on the same footing; 2) envoys extraordinary, ministers plenipotentiary and internuncios; 3) ministers resident (a class added at the Congress of Aix-la-Chapelle); 4) chargés d'affaires: the envoys of the first three classes are accredited to the Head of State, the chargés d'affaires to the minister of Foreign Affairs. According to the Regulation of Vienna (art. 2) only the first category has the representative character: but this diversity, which derives from the quality of personal representatives of the Sovereign originally attributed to ambassadors, is understood in the sense that only ambassadors represent, in addition to the State, also the person of the accrediting Head of State, and it translates only into ceremonial prerogatives. The agents of the categories listed above are only the heads of diplomatic representations, whose members have increased and are increasing in number and variety of attributions in step with the development of international relations. Normally a representation is composed, besides the head, of counsellors, secretaries and attachés of legation, of chancellors and of other official personnel.
The diplomatic agents accredited to a State form the diplomatic corps, at the head of which, by tradition and express provision of the protocol of Vienna, is the pontifical nuncio or the highest in rank or most senior of the lay agents. The only substantial function of the diplomatic corps is to watch over the respect, on the part of the local government, of the immunities and privileges pertaining to its members. The diplomatic mission, a term that designates the period during which the diplomatic agent exercises his functions in a State or in the Holy See, officially begins after the two States have manifested the desire respectively to send and receive the diplomatic agent, with the presentation of the letters of credence (v.), preceded, for the first three classes of agents, by the agrément (v.), or "approval" of the accredited State; it ends with the completion of the mission (envoy extraordinary); with the recall by the accrediting State; with the rupture of diplomatic relations; with the arising of a state of war between the two States; or for reasons of a personal or administrative character that determine the replacement of the envoy.
During the period of their stay in the accrediting State, diplomatic agents enjoy special immunities and privileges (v.) intended to facilitate the carrying out of their mission.