When does a good man go?

ONCE more, in a country with close ties to the United States, political change has occurred and a friendly ruler has been set aside. The former President of Tunisia, Habib Bourguiba, is today under a form of house arrest and a new President, Zine al-Abidine Ben Ali, governs in his place. In the event, one can find both similarities to changes in other countries and substantial differences. The similarity lies in the repetition of the age-old drama of the leader who has stayed too long. The US has observed this in the rule of Haile Selassie in Ethiopia and King Idris in Libya.

Aging rulers do not like to step down. Neither do they like to designate a successor; they fear that they will become increasingly irrelevant as courtiers turn to the heir, not to the king. To preserve their power, as Mr. Bourguiba did in his later years, they make frequent changes in the government, creating uncertainty, if not fear, in those around them. Their suspicion often extends particularly to members of their own family.

Haile Selassie was always suspicious of his son, the crown prince. King Idris gave little rein to his nephew and heir. Bourguiba's relationship with his son, Habib Jr., has always been difficult.

As long as 15 years ago, visitors to Bourguiba detected momentary losses of lucidity. Over the years the condition grew worse. Declaring himself ``President for Life,'' Bourguiba became more and more arbitrary, his periods of awareness more and more infrequent. As in other lands, the growing senility and arbitrariness of the ruler created political unrest and tensions, even in a country as relatively homogeneous and with as strong a social fabric as Tunisia.

Although this common human drama that has just been played out in Tunis may have similarities to those in other lands, Tunisia is fortunate in that the end to Bourguiba's rule came not, as in Ethiopia and Libya, in a revolutionary coup, but in an orderly change.

Tunisia's Constitution provides that the prime minister will succeed the president until the next National Assembly election.

Prime Minister Ben Ali has recognized this and has buttressed his legitimacy by the report of a panel of doctors on Bourguiba's health. He has, moreover, in the process of change, shown respect for the former President and his contribution to the country.

Such respect is merited, for Bourguiba has been a remarkable leader. After a long struggle with the French, he led his country to independence in 1956. Tunisia is not rich in resources, but under Bourguiba's leadership it has had decades of relative prosperity, compared with many other newly independent countries.

In the political realm, Bourguiba has been shrewd and independent. He provided a haven for Algerians fighting for independence while at the same time building a satisfactory relationship with France. Taking issue with the often strident rhetoric of other Arab leaders, he urged a recognition of the reality of the existence of Israel many years ago. By careful diplomacy, he has held at bay the ambitions of his neighbor, Muammar Qaddafi. Bourguiba has always been a close friend of the US. To every American visitor, he has recounted his debt of gratitude to a US diplomat, Hooker Doolittle, who, during World War II, helped Bourguiba escape from the French; at that time Paris considered this young North African, with his demands for independence, a dangerous threat. In later years, even though Tunisia's geographic position and Bourguiba's independent policy precluded any close security alliance with the US, Washington has provided substantial economic help and some military help.

Inevitably the fortunes of the US rest on the character and future of rulers in countries with which Washington has close relations. Too often in the past US interests have been undermined by political change that has altered radically the orientation of a country. Inevitably in such cases, the question has arisen of why US diplomats cannot have more influence in urging friendly rulers to prepare for the end of their rule. Time and again, efforts have been made, but rulers and, especially, aging rulers, do not like to contemplate the end.

The US should welcome, therefore, an orderly change carried out by those in Tunis in recognition of the frailty of an old man but, at the same time, in a manner that preserves the tradition and orientation of this important North African country.

David D. Newsom is associate dean and director of the Institute for the Study of Diplomacy at Georgetown University.