By Danny Wood
BBC News, Madrid
The arrest of Eta's suspected military chief, Mikel Garikoitz Aspiazu Rubina, means that in the last six months the Spanish and French authorities have captured two men they describe as key leaders of the militant Basque separatist group.
Mr Garikoitz Aspiazu was arrested in the French town of Cauterets
In May, Javier Lopez Pena, a man described by the police as the most senior Eta commander, was arrested in Bordeaux.
At the time, Spain's Interior Minister Alfredo Perez Rubalcaba said Mr Lopez Pena was "in all probability, the person who carried the most weight within Eta - politically and militarily".
But Eta's attacks continued. In September a car bomb exploded near a military academy in northern Spain killing an army officer and injuring several others.
That attack came just hours after two other bombings that injured 11 people. In October, a bomb blast at a university in Pamplona left more than ten people hurt.
In fact, in the last six years, five people described as military and political leaders of the militant Basque separatist group have been arrested.
In a similar vein to the announcements made after previous arrests, Spanish Prime Minister Luis Rodriguez Zapatero said the detention of Mr Garikoitz Aspiazu was a "decisive" blow.
Perhaps because of the hyperbolic way their political leaders announce these detentions, many Spaniards will probably not expect this latest arrest to have much effect on the frequency of Eta's bomb attacks.
But the authorities and anti-terrorism specialists are sure that Mr Garikoitz Aspiazu's arrest is a serious setback for the armed separatists, especially for its hardliners.
"He's a symbol for the hardliners in the band," said Alberto Surio, a correspondent based in the Basque region who specialises in militant separatism.
"The group that dynamited the last peace process and the element whose answer has been terrorism during a long cycle of violence - that is now almost certainly neutralised and deactivated because of this arrest," he told Spanish public television.
Mr Garikoitz Aspiazu was the leading representative of a new generation of Eta militants characterised by their extremism, lack of preparation and a tendency to regard violence as the only way of achieving the objective of an independent Basque nation.
In recent years he is thought to have had increasing influence over the armed group, directing many of its attacks during the last decade including the fatal bomb attack on Madrid's airport that ended the last Eta ceasefire in December 2006.
Some Basque specialists say that in spite of previous exaggerated statements by the authorities about other detentions, this arrest is particularly important.
"It's very significant. Garikoitz Aspiazu was running the part of Eta that actually works, because the rest of it is in disarray," said Spanish political analyst Miguel Morado.
"At the moment nothing is working in Eta - every time they go out and do an attack they are arrested," he added.
"What he did was separate different cells and make them all answerable to him. So he created an Eta within an Eta and this was the Eta that actually functioned effectively," he said.
Although officially not the top Eta leader, Miguel Morado says Mr Garikoitz Aspiazu was the most important person in the organisation.
The French authorities say they plan to send him to Spain for trial after completing other police operations connected with his detention.
The arrest of Mr Garikoitz Aspiazu is very bad news for Eta and could see a decline in the influence of its new generation of hardliners.
But at the moment, no-one expects Eta to change its strategy. The armed organisation seems intent on sticking to what it knows best: violence.