300 years of diplomatic immunity
The taking of hostages, using humane feelings for blackmail, has unhappily become endemic in some parts of the world; but it is barbarity without precedent in modern history for a government to do this. It is even graver that a sovereign state maltreats diplomatic personnel like a terroristic gang. The inviolability of ambassadors is the oldest principles of international law and for centuries the best observed.
The ancient Greeks usually respected the immunity of envoys, and this was fixed in Roman law. The Middle Ages were less punctilious, but as modern nations grew up it became conventional to treat ambassadors as representatives of sovereigns, with due dignity regardless of feelings toward them.
For example, in 1562 the British ambassador to France was allowed to leave freely despite conniving with the king's enemies. In 1584 the British found the Spanish ambassador involved in a plot to kill Queen Elizabeth and replace her by Mary Queen of Scots. After consultation with a learned commission the government did not punish but simply expelled him.
In 1625, Hugo Grotius, so-called father of international law, wrote of "two points with regard to ambassadors which are everywhere admitted as prescribed by the law of nations, first that they be admitted, and then that they be not violated."
Since the beginning of the 17th century there has been debate on the extent of the immunity of diplomats and their quarters, not on the principle itself. For example, in 1660 the French envoy to the papacy claimed immunity from arrest not only for his own suite but for persons residing in his vicinity. From time to time, the granting of asylum in embassies to outlaws has been questioned, and as late as the 18th century the Sultan of Turkey sometimes manhandled Christian envoys. But it has been recognized over 300 years by all civilized states that the sanctity of diplomatic personnel is essential for the conduct of international relations.