The Israeli raid on a Jericho jail last week, which ended with two dead Palestinians, the capture of six prisoners who were about to be set free by the new Hamas government, and the ignominious surrender of dozens of Palestinian policemen stripped to their underwear to make sure their bodies were not booby-trapped, was a trivial episode in Israel’s conflict with the Palestinian Authority. It was, however, yet another lesson in the futility of third-party intervention in situations of national conflict.

The captured men – five of them convicted, in a rare Palestinian trial of terrorists, of assassinating Israeli cabinet minister Rehav’am Ze’evi – were imprisoned as part of an American-brokered deal that ended a 2002 Israeli siege of Yasser Arafat’s Ramallah compound, in which they had taken refuge. The agreement was that they would be sentenced to incarceration in Jericho, with U.S. and British observers posted in the jail, where they reportedly enjoyed deluxe conditions, to make sure they didn’t “escape.” It was last week’s hasty departure of these observers, concerned for their safety under a Hamas government, that led to the Israeli action.

In a word, as long as the Palestinian Authority was committed, however half-heartedly, to keeping the six men in prison, the observers were unnecessarily stationed outside their jail cells. (One hopes they at least brought with them some comfortable chairs and a supply of good books.) The minute there was a need for the observers they vanished, obeying instructions from their superiors.

“Well, what else could we have done?” these superiors would no doubt say. “Risk our personnel to carry out a job that had become dangerous?”

And yet this is exactly what they should have done! Quite apart from the danger not having been very great (the last thing a Hamas government would have wanted would be to be responsible for the death or injury of American and British representatives), what were the observers there for in the first place if not to do their best to prevent the convicted men from walking out of prison? At the very least, they should have remained at their posts long enough to offer token resistance to the prisoners’ being freed and to protest officially in the name of those who sent them.

Alas, such is the almost inevitable fate of international interventions of this kind. They work perfectly when they are superfluous and collapse the minute they are not.

There were Palestinians last week who, convinced that there was Anglo-American-Israeli collusion behind the prisoners’ capture, rather absurdly compared it to the Anglo-French-Israeli collusion of the 1956 Sinai Campaign against Egypt. But if there was any valid comparison to Sinai, it was an entirely different one. This was the similarity (on a far smaller scale, needless to say) of the observers’ departure from Jericho to that of the U.N. peacekeeping force that fled Sinai in May 1967, paving the way for the Six Day War.

This U.N. force, too, was part of an international agreement, one according to which Israel withdrew its army from Sinai in return for a U.N. guarantee that Egypt would honor the right to free passage of ships bound through the Straits of Tiran for the Israeli port of Eilat. And indeed, the straits remained open for 11 years – for as long, that is, as the Egyptians were content to let them be so. The minute Egypt changed its mind, the guarantee evaporated and the U.N. force vanished.

Alas, such has been the world’s experience with practically every international presence that has tried to intervene in interand intra-state conflicts without taking sides in them: It happened with the U.N. troops deployed in the Congo in 1960, who failed to protect the legally elected government of Patrick Lumumba; with the U.N. force sent to Cyprus in 1964, which did nothing to prevent the 1974 Turkish invasion of that island; with UNIFIL in Lebanon, under whose eyes the Hezbollah has operated with impunity for years; with the NATO contingents in Bosnia that ran from Srebrenica in 1995, and in still other cases.

This is not, really, very surprising. Governments are willing to endanger their soldiers, and soldiers are willing to endanger themselves, when they are fighting an enemy that is defined as such and that is in turn trying to harm or kill them. International peacekeeping missions that have such enemies, such as the 1999 NATO expedition to Kosovo, which was essentially an anti-Serbian operation, stand a chance of succeeding.

But governments and soldiers will not fight when they view themselves as neutral parties, called upon to intercede between states or armies that they do not consider hostile to themselves. Psychologically, this is natural. Why should the U.N. forces in Sinai in 1967 have risked casualties to do battle with Egyptian troops they had nothing against? Why should the Dutch have died at Srebrenica defending Bosnian Muslims they were not allied with? Why should American or British observers in Jericho be asked to face even minor unpleasantness when the conflict between Israel and the Palestinian Authority is not one they are a side to?

For Israel, the lesson is clear. Neutral “peacekeepers” between it and the Arab states are always undesirable. They create an illusion of false security, solve no real problems, can never be counted on in a pinch, and only tend to get in the way when military action is called for. They are about as useful as an umbrella in clear weather, and about as much of a deterrent to rainstorms when they come.

Mr. Halkin is a contributing editor of The New York Sun.