INTERESTING CASE, Tunisia: a moderate place and an old friend of the West that fears, with reason, that its neighbor Libya wishes to do it in. The other day, a party of anti-government rebels apparently drove across the porous border and attacked the city of Gafsa. Whether the attackers intended to set up a paper regime and seek Libyan intervention or simply to destabilize, Mecca-style, President Habib Bourguiba is uncertain. The Tunisian army, perhaps with a little discreet French help, carried the day. But the Tunisians were shaken. Libya has a far more powerful (Soviet-built) army.It has a leader, Col. Muammar Qaddafi, with an outlaw mentality and a long record of hostility toward Tunisia. It has a radical ideology that is a potential spark in Tunisia's inflammable social tinder.

In ordinary circumstances, the United States -- particularly the Carter administration -- faced with an urgent Tunisian appeal for at least a symbolic show of comradeship, would have taken the request under diligent advisement for some months, if not longer. It would have been observed that the Gafsa incident took place on the second anniversary of a traumatic general strike, an event that not only dramatizd the country's unmet social and economic needs but also led the government to take certain steps that have troubled human-rights activists since. President-for-life Bourguiba, it would have been realized, though he has performed good service to his country and to the cause of regional moderation in the past, is 79, sick and slowing, and has failed to modernize his country's independence-era political structure. The resemblance, such as it is, of his situation in Tunisia to that of the shah in Iran and the royal family in Saudi Arabia would have been widely remarked.

In fact, the United States received and is acting on a Tunisian request for military assistance in a matter of days. The evident feeling is that, after Iran and Afghanistan, this is no time to conduct a probing sociological inquest and thereby to convey an impression that when longtime friends get in trouble, the United States takes its good time in coming to their aid. Among countries outside the region, it falls naturally to France to provide the most substantial reassurances to its erstwhile colony, but the United States has a role. Fortunately, the administration is playing it.