In high school textbook theory the United States never intervenes in the internal affairs of other countries.
In practice it has done so scores of times and is doing so again right now, in El Salvador, up to the limit of tolerance of the Congress and public opinion.
The temptation to intervene is sometimes enormous, but the record of results would seem to indicate that the urge should be resisted as much as possible and indulged only when the reasons seem to be overriding.
Much of the current trouble in Central America is traceable back to earlier US interventions. Usually such intervention seems to have produced a short-term success, but with a long-term aftertaste of local resentment.
Nicaragua is a case in point. It is now in the hands of a revolutionary government which enjoys close relations with Fidel Castro's Cuba and is believed in Washington to be getting arms and other forms of support from Moscow.
The US has been in and out of Nicaragua beginning in 1909. In that year the US drove out of office a popular and moderately successful (as such things go) government headed by a local politician named Jose Santos Zelaya. Senor Santos was suspected in Washington of thinking of selling to the British a concession to dig another transisthmian canal through Nicaragua. Washington did not want the British to have their own canal rivaling the American Panama Canal. Out went Senor Santos.
By 1912 there was much unresolved unrest in the aftermath. US marines were sent in backing a faction which preferred taking US to British money. The marines remained in from 1912 to 1933 with a brief intermission in 1925. During their stay they trained a National Guard. When the marines finally left in 1933 the commander of that military unit was one Anastasio Somoza Debayle whose henchman proceeded to assassinate Cesar Augusto Sandino, leader of a reform political faction with considerable popular support.
From that time Nicaraguan politics were influenced by the Somoza family on the right and the heirs of the Sandino group on the left. Popular opposition to the Somoza regime reached the boiling point in 1979. The Sandinistas came in with a strong anti-US bias since the US had for so long supported and traded with the Somoza regime. The Somozas went to Miami where their money now funds the training of a right-wing military force.
Considering the background it is not surprising that the Sandinistas look elsewhere than to Washington for friendship and arms.
Guatemala is another case where the US intervened. It happened in 1954. The US became concerned over the left-leaning tendencies of the reformist government of Col. Jacobo Arbenz Guzman. He had launched a land distribution program including land belonging to the United Fruit Company. A shipload of weapons was reported approaching Guatemala. The CIA was authorized to support a right-wing rebel force. It invaded the country from Honduras and El Salvador just ahead of the ship with the weapons for Senor Arbenz. The US thus associated itself with the right-wing cause in Guatemala, which is now thoroughly polarized between a hard right government and a peasantry in chronic rebellion. Guatemala government forces are said to have killed 13,000 peasants since 1978 and to be killing now at the rate of 300 a month. The opposition tends to look elsewhere than to the US for help.
Iran is another case, like Guatemala, where US intervention produced quick and successful short-term results. That was also a CIA operation.
In 1951 Prime Minister Mohammed Mossadegh had nationalized the oil fields to the dismay of British and American oil interests. The British blockaded the coast. The Shah was forced out. The economy went sour. By 1953 times seemed propitious for a counterrevolution. A single CIA agent went in with a suitcase full of money, hired a street mob, and overthrew Mossadegh. It was really almost as easy as that. The Shah came back and ruled the country for the next 26 years, in happy collaboration with the US.
But in 1979 the fundamentalist Muslim movement toppled the Shah and tossed out his American friends. The episode of the US Embassy hostages followed. The US was so deeply mixed up with the Shah's regime that there are still no formal diplomatic relations (or any other kind) with the present Islamic regime.
Washington has not always picked the losing side when it intervenes in other countries' affairs. But there seems to be a tendency in that direction. The US made good its support for South Koreans, but its interests went down with the losers in the Chinese revolution of 1949 and in Vietnam.