INTERVENTION (Lat. intereentio, inter position, a coming between, from intereenire, to ecane between, from inter, between + venire, to come). (1) An act or proceeding by which a person not originally a party to an action is ad mitted as such for the protection of a right or interest in the subject matter of the litigation, which will be materially affected by its deter mination. It has always been a well-established right under the civil law and was early intro duced into the English procedure. In a few of the United States the right is confined to actions involving real or personal property, but in most jurisdictions no such limitation exists. A person may be thus admitted as a party plaintiff or de fendant, by application to the court before the cause has proceeded to trial. See PLEADING; PRACTICE; PARTIES.
(2) In international law, the interference of one State in the affairs of another for the purpose of preventing injury to itself, or in behalf of some principle, person, or collection of persons. Intervention may assume several forms. Thus it may be by verbal note delivered to the ambas sador of the country against which the grievance is held; it may be by ollieial notes publicly de clared; it may take the form of an international congress or conference; or, finally, it may result in armed interference. It is the last form which is generally understood when reference is made to acts of intervention. Notable instances of intervention have been the invasion of Holland by the Prussians in 1787 to restore to his old position the Stadtholder, who was a brother-in law of the Prussian King; the interference of the Holy Alliance to thwart liberal movements, and restore absolutism in Spain, Naples, Sicily, and Piedmont in the early part of the nineteenth century; the interference of Austria and Prussia in behalf of Louis XVI. of France in 1791: and the intervention of France in Rome in 1849 to restore the temporal power of the Pope.
No general rule can he laid down to determine when intervention is justifiable. It is well settled that it is excusable as a measure of self-preserva tion. Thus, in Europe, the policy of interference in behalf of the balance of power has been quite common. Any European State may he restrained from pursuing plans of acquisition of territory.
Europe is regarded as a confederacy of Slates. and the balance of power is the guarantee of na tional existence against the designs of the more powerful States. This principle is not violated by territorial uggrandizcments. so long as the safety and liberty of neighboring States are not endangered. Notable instances of intervention under this head were that against Louis XIV. to prevent him from seating his grandson on the throne of Spain ; the coalitions against Napoleon: the interposition of the four Powers in 1840 in favor of the Sultan against Mehemet Ali of Egypt; and that of France and England in 1854 to preserve the integrity of the Turkish domin ions against the designs of Russia. Concerted ac tion or intervention by the great Powers of Eu rope is sometimes resorted to in order to prevent a smaller State from following a particular line of conduct or to compel it to do a certain act. An instance of this form of intervention was the formal recognition of the independence of Belgium by the five great Powers in 1830, when that country emancipated itself from Holland. With the development of modern civilization, humanitarian motives are often appealed to a, a justification for interference where cruelty is in flicted by despotism or religious intolerance. Such cages must be carefully scrutinized, for the danger always exists that they may cover ambi tious projects for territorial aggrandizement. But interventions on the ground of humanity have under exceptional circumstances certainly a moral, if not a legal justification. Such was the intervention of Great Britain, France, and Rus sia. in 1827. by which Greece was liberated from Turkey, and again in the interposition of the great Powers in 1860 to stop the persecution and massacre of Christians of the Lebanon. The most important recent instance of intervention is that of the United States in Cuba in 1S9S, which may be referred in part to humanitarian feelings aroused by the cruelties attending the insurrectionary struggle against Spanish author ity, in part to the destruction of American prop erty, and the menace to American interests re sulting from the possible acquisition of the island by another foreign power. See INTERNA TIONAL and the authorities there referred to.