Intervention is as ancient and well-established an instrument of foreign policy as are diplomatic pressure, negotiations and war. From the time of the ancient Greeks to this day, some states have found it advantageous to intervene in the affairs of other states on behalf of their own interests and against the latters' will. Other states, in view of their interests, have opposed such interventions and have intervened on behalf of theirs.
It is only since the French Revolution of 1789 and the rise of the nation- state that the legitimacy of intervention has been questioned. Article 119 of the French Constitution of 1793 declared that the French people "do not interfere in the domestic affairs of other nations and will not tolerate interference by other nations in their affairs." This declaration ushered in a period of interventions by all concerned on the largest possible scale. For a century and a half afterwards, statesmen, lawyers and political writers tried in vain to formulate objective criteria by which to distinguish between legitimate and illegitimate intervention. The principle of nonintervention was incorporated into the textbooks of international law, and statesmen have never ceased to pay lip service to it. In December 1965, the United Nations General Assembly adopted a "Declaration on the Inadmissibility of Intervention in the Domestic Affairs of States and the Protection of their Independence and Sovereignty," according to which "no state has the right to intervene, directly or indirectly, for any reason whatever, in the internal or external affairs of any other state . . ." and "no state shall organize, assist, foment, finance, incite or tolerate subversive, terrorist or armed activities directed toward the violent overthrow of another state, or interfere in civil strife in another state." Yet again we are witnessing throughout the world activities violating all the rules laid down in this Declaration.