In the history of international terrorism, the Iranian crisis is unique: never before has a government so openly supported an act of terrorist extortion by its own citizens against another nation.

But in the view of some experts, it was an event certain to happen. They think that, as conventional and nuclear warfare become more frightening and less practical, terrorism may become a form of "surrogate warfare" for volatile Third World nations to achieve their political ends.

A 1976 Rand Corp, study for the State Department predicted as much: "Some nations in the future may employ terrorist groups as a mode of surrogate warfare. Although we may foresee an era of formal peace between nations, at the same time we may be entering an era of increased political violence at lower levels."

Iran is the latest and most dramatic confirmation of this prediction. Other ominous incidents have included the 1976 murder here of former Chilean ambassador Orlando Letelier by the Chilean government's hired assassins and the 1975 terrorist takeover, instigated by Libyan chief Muhammar Qaddafi, of a meeting of the Organization of Petroleum Exporting Countries.

Terrorism has apparently become a weapon not only of isolated extremist groups but also of nations.

"This represents a domain of conflict in the future," said Brian Jenkins, chief of Rand's Research Program on Subnational Conflict and author of the 1976 study. "We may well see, in the coming decade, fundamental changes in the nature of world order and in the rules which have governed the world to date."

Anthony Quainton, director of the State Department's Office for Combating Terrorism, notes: "Iran isn't the traditional terrorism by small groups, or a state terrorism against its own people. It's the use of terrorism by a state against an outside power. That's a new use of old tactics. It's different from what we've seen in the past."

He said the use of terrorism as surrogate warfare "is much discussed in the small world of terrorism experts. Is terrorism a strategic weapon or is it likely to become one? That's a very speculative area, but it deserves serious intellectual analysis."

The State Department is also concerned, he said, about an increase in mob action against embassies Yesterday's sacking of the U.S. Embassy in Islanabad, Pakistan, follows incidents in the past against embassies in Stockholm, Kuala Lumpur, Madrid, and San Salvador.

Foreign Service families are trained in coping with terrorism, and State Department officials are put in hypothetical situations to sharpen their decision-making skills in the event of a terrorist attack.

The United States has led efforts to enforce two international conventions -- one on hijacking and the other on safety for diplomats. The latter, signed by 42 nations, including Iran, contains no sanctions against countries that do not comply.

Dr. Frank Ochberg, an internationally known expert on terrorism, emphasizes the unique aspects of the Iranian crisis. "Terrorists usually operate on hostile soil. In this case, you don't have the outer perimeter of police trying to rescue the hostages. Instead, they [terrorists] are getting all kinds of support from the community."

Ochberg, who worked with Dutch citizens taken hostage by South Moluccans in 1977, has studied the psychological aspects of terrorism. He says hostages often develop strong, positive feelings toward their captors.

The phenomenon, known as the Stockholm sydrome, was first noted six years ago when hostages held for six days following a bank robbery found they did not resent their captors. One hostage even felt they were protecting her from the police.

The syndrome may explain why Marine Sgt. William E. Quarles of Washington said after his release by his Iranian captors that he had been treated well and "made many good friends" among them.

"It doesn't matter how well or how badly you're treated," Ochberg said. "A hostage is reduced to an infantile state.It's as though an individual taps into a wellspring of very early feeling -- like what an infant feels when he is relieved of the pain or terror of hunger, cold or isolation. It is a primitive gratitude for not being killed."

Ochberg said hostages are rarely killed after the third day of captivity, partly because captors also come to sympathize with their prisoners. Most deaths in such incidents result from liberation attempts. According to Jenkins, 86 percent of all hostages survive the ordeal.

Jenkins said that in the last few decades terrorism has claimed fewer than 2,000 lives.

"The best strategy in most cases is to sit and wait until the hostage-takers tire and the Stockholm syndrome takes over," Ochberg said.

"The notion of waiting it out has been accepted as part of the conventional wisdom. Ultimately the captors become weary and fold," Jenkins said. However, one expert on terrorism noted that the Iranian militants might find it hard to give in even if they wanted to because of the chanting mobs outside the Tehran embassy and the resistance of Iran's leader, Ayatollah Ruhollah Khomeini.

Hostage-taking has become the second most popular terrorist tactic, after bombings, Jenkins said. "Hostages are more dramatic than murders," he added. "Human life hangs in the balance. A murder or bombings is in the news only one or two days. A hostage situation can last weeks."