How do you end a terrorist war? For anyone thinking of writing a user's handbook, here are some suggestions. First, you must want to. The most positive aspect of the permanent ceasefire announced this week by Eta Basque separatists is that there's a widely shared political will for the peace process to succeed.

This is new. It's essential, because in the long, difficult process that lies ahead, there are bound to be setbacks: Basque radicals may lurch out of control of an organisation suddenly committed to talks. Others may baulk at sitting down with those responsible for sowing death and fear among their friends and family. So you've got to want to keep trying.

Previous attempts to negotiate Eta ceasefires, in the 1980s under Felipe Gonzalez's socialists and in the 1990s during the conservative government of Jose Maria Aznar, foundered on the twin reefs of mistrust and clumsiness. That was because neither side trusted the other's good faith. They hadn't prepared the ground.

But how can you prepare the ground for something which is supposed to launch a new process? Well, you have to open doors, and you have to involve everyone, not just parties closest to those who until yesterday were your enemy. Eta's last ceasefire, in 1998, which lasted 14 months, was pacted with the region's ruling Basque Nationalist Party, a majority within the Basque country, but only just. A large minority of Basque parties, and most Spaniards, were never convinced. Many believed Eta took advantage of the reprieve to rearm.

It helps if your terrorists are weakened. That's achieved partly by good intelligence and fierce policing. Jose Luis Rodriguez Zapatero has been as implacable in his anti-terror campaign since he came to power two years ago as Mr Aznar was. Scores of Eta leaders have been detained, weakening and demoralising the untried youngsters who stepped into their shoes. Numerous terror attacks were foiled.

But this government recognised - without shouting it to the rooftops - that Eta could never be destroyed by police or military means. Mr Zapatero's socialists absorbed the lesson that the Basque conflict, like most terrorist wars, is political, not just military, and requires a political solution. Eta still has a social support that can produce up to 20 per cent of the popular vote. You cannot crush an organisation that enjoys unconditional support from the families and friends of hundreds of political prisoners dispersed in far corners of Spain, and from entire towns who for decades have returned councillors pledged to independence and self-determination.

From the moment Zapatero came to power, he started preparing the political ground. He opened the door, without making concessions, with a clear intention of bringing Eta in from the cold and solving the conflict for good. He said he would talk if Eta laid down arms, but only on that condition. Aznar had said substantially the same thing, but he made it sound a threat ("stop shooting and bombing or we will never talk"). Zapatero, by contrast, made his offer sound more like a promise: we are prepared to talk, if we can get the bombs and bullets put to one side.

Tone is very important in ending a terror war. Zapatero's words are softer than Aznar's, but he has conceded nothing. Batasuna has constantly asked for the government to make a move, making it possible for the armed men and women to emerge from the trenches. Zapatero hasn't budged.

He's been preparing. He's deployed his sappers and intermediaries to sound out the enemy and establish what terms might lead to a cessation of hostilities. Even before Zapatero came to power, Basque socialists and parties close to the nationalist cause were making contacts with priests, trade unionists and even Basque businessmen who might have the ear of Eta. His government consolidated the process, but with infinite caution, so he was never directly involved: "The government has no contact with terrorists."

So when beady journalists asked the relevant minister whether, off the record, he could confirm that members of his party in the Basque country had contacted those close to Eta, he could say, "You know I have to say no, even if they were, so the answer is no." The pre-ceasefire has to be deniable.

Finally, it helps to have a cruel example close to home. After the 11 March Madrid bombings that killed 192, no one in their right mind could imagine that acts of terror could advance the cause of Basque independence. Eta recognised that the public would spurn them unless they presented a peaceful alternative. And Zapatero is ready to listen.

e.nash@independent.co.uk