Basque terrorists on defensive. French-Spanish combined action pressures radicals to surrender. COOPERATING ON SECURITY

Basque terrorism seems to have been defeated. French police stormed a villa near Bayonne in southwestern France over the weekend and captured Philippe Bidart, leader of the French Basque separatist organization Iparretarrek, along with four accomplices.

At the same time, the Spanish government confirmed that it is working to negotiate the surrender of the isolated Spanish Basque separatist organization, ETA (an acronym for Basque Homeland and Liberty).

Cooperation between France and Spain has put the terrorists on the defensive. For years, France offered refuge to Basque nationalists it said were persecuted by the authoritarian regime of Gen. Francisco Franco.

But Spain now is a democracy and attitudes have changed.

French police work with their Spanish counterparts, helping them track down suspected terrorists. Suspected ETA members regularly are extradited and suspected French sympathizers are jailed.

In Spain, Basque separatism and terrorism took deep roots during Franco's oppressive reign. As revenge for the Basques' support of the Republic during the Spanish civil war, Franco outlawed the Basque language and banned public meetings. The Basques responded in 1959 by creating ETA.

By contrast, Spain's new democratic rulers have moved to satisfy the aspirations of Basques, who mainly inhabit a region west of the Pyrenees, along the French-Spanish border. A decade ago, they granted Basques a large degree of autonomy, including powers over taxation, television, and education. For the first time, the most radical Basque party, Herri Batasuna, agreed last year in a parliamentary vote to come out and condemn terrorism.

ETA, whose independence campaign has taken nearly 600 lives, found itself isolated. On Jan. 29, the group offered a 60-day partial truce if the Spanish government agreed to resume contact with members of its leadership exiled in Algeria. What was novel in ETA's statement, most experts agree, was its offer to halt the violence without obtaining any prior commitments from the Madrid government.

At first, Madrid hesitated. Authorities were angered by a particularly nasty ETA car bombing in December which killed 12 people in a civil guard barracks in Saragossa.

Government spokesman Javier Solana said that the decision to accept the ETA offer was taken ``in the light of the fact that there have been no killings'' in the past few weeks.

In France, Basque separatism always has been less of a threat than in Spain. French Basque lands are smaller and less populated than Spanish Basque lands; French Basques integrated into France and even gave France its familiar cap - the beret. ``We're French first,'' says Guy Chardiet, tourism director in St. Jean de Luz. ``We eat Camembert and baguettes, too.''

In this Francophile atmosphere, analysts agree violence never enjoyed widespread local support although Iparretarrek began in the 1970s as an ETA offshoot. Members helped their Spanish compatriots. Their own actions remained symbolic - actions such as sabatoging the Tour de France bicycling race.

No one was hurt until 1982 when two terrorists were killed by their own bomb. An attempted kidnap in 1982 ended in the murder of two police officers. French Basque leader Bidart was convicted last year of this crime in absentia and as a result was sentenced to life in prison.

``This is a big step in dismantling a terrorist network,'' Security Minister Robert Pandraud said after Mr. Bidart's arrest. ``I believe Ipparretarak will have a hard time regrouping and starting over.''

A larger lesson may be gleaned from this success. Two years ago, an international team of security experts studied the Basque problem and concluded in a 250-page report that Basque terrorism could be beaten by a combination of politics and policing.

On the one hand, the experts said that political causes behind the violence must be addressed through negotiations with ETA. On the other hand, police should put pressure on the terrorists ``to show that violence has no chance of succeeding,'' according to Jacques L'eaut'e, one of the report's authors.

By opening negotiations with ETA, the Spanish government has followed these recommendations. Reports from Madrid suggest it may even be prepared to make further concessions on Basque autonomy - but not on independence - in order to silence ETA.

But Mr. L'eaut'e is optimistic about ending the Basque threat.

``Basques share Spanish ethnicity and the Catholic religion,'' he concluded. ``Basque demands can be reconciled in today's Spain.''