Once countries have established diplomatic relations, the next step will be the appointment of a head of the diplomatic mission (usually to the rank of ambassador). Diplomatic law requires prior authorization, called accreditation, for the appointment of a particular person to the position of head of mission. The application for accreditation is made through diplomatic channels. The state in question may refuse to give its consent and is not required to justify its decision. The accepted candidate, after being appointed head of mission according to the procedure of his own country, arrives in the host country with letters of authentication (in the case of Chargé d`affaires, these will be introductory letters). The official start of his duties as Head of Mission is the time of accreditation, i.e. the filing of his authentication letters (introduction). Any state that is the subject of international law has what is called a right to legitimacy (ius legationis), i.e. the possibility of participating in international relations by sending its own diplomatic representatives to other states (the right to active legislation) and to welcome foreign diplomats on its own territory (right to passive legitimacy). However, the establishment of diplomatic relations requires the mutual agreement of the States concerned. The practice and theory of diplomatic law have not developed generally accepted criteria for refusing agrement. However, three groups of general criteria can be separated: the first relates to the candidate himself, his past and his attitude towards the host country; the second is due to the state of relations between the two countries and therefore does not directly concern the person of the candidate for the position of Head of Mission.
Thus, the United Kingdom refused to accept the clergymen as representatives of the Holy See. The same is true for countries that do not want to recognize the state or the government itself or do not want the person concerned to be accredited simultaneously in certain countries, such as the United States.B. Italy and the Vatican do not agree that the same person should represent a state in both Italy and the Holy See. The third reason for the attacker`s refusal may be the question of the candidate`s nationality, particularly if he or she has the nationality of the host state. In the past, this problem has been solved differently. Some countries agreed, others did not, or exceptionally, as in 1868, when the United States refused to accept a Chinese ambassador of American nationality. In 1945, however, the U.S. government agreed to appoint Oskar Lange as ambassador of the Polish provisional government, even though he was a U.S.
citizen at the time. However, in international practice, the principle of non-aggressiveness prevails for its citizens and, to a lesser extent, for naturalized expatriates.