The military leader of the Basque terrorist group ETA was removed from the top of Spain's most-wanted list early Monday morning, when French police arrested Garikoitz Aspiazu Rubina, alias Txeroki, in the Pyrenean town of Cauterets. Spanish police allege that he's directly responsible for several murders. Txeroki, they say, is suspected of ordering the 2006 bombing at Madrid's Barajas airport.

"This has been a hard blow and ETA is going to suffer for it," said Spanish Prime Minister José Luis Rodríguez in reference to the arrest. "Today ETA is weaker and our democracy is stronger."

Although experts have been sounding the death-knell of ETA for years, the detention of Txeroki comes on the heels of several other significant arrests, including the recent capture of the group's political leader. The separatist group, which has killed more than 800 people in the past 40 years, is now believed to be in a historically unprecedented state of decline.

Txeroki, who is in his 30s, had only recently risen through the ranks of ETA's hierarchy, replacing previous operational leaders Mikel Albizu and Soledad Iparraguirre, who were arrested in 2004. From the outset, he showed himself to be a hard-line partisan of violence as the preferred means of obtaining an independent Basque homeland. Early in his leadership, police allege that he ordered an assassination attempt on the life of Spanish king, Juan Carlos.

"He is the hardest of hard-liners," says sociologist Ignacio Sánchez-Cuenca, of Madrid's Juan March Institute. "This is a person deeply implicated in violence."

Two members of ETA now behind bars have also testified that Txeroki himself pulled the trigger on two Spanish Civil Guards assassinated in the French coastal town of Capbreton earlier this year. Perhaps the most serious act of his career, however, was the bombing of a parking lot at Barajas airport in December 2006. In addition to killing two Ecuadorean immigrants, the attack brought an abrupt end to flailing peace talks with the Spanish government that began when ETA declared a "permanent" cease-fire in March of that year. Today, the headline of a profile of Txeroki in Spanish newspaper El País declared: "The ETA member who blew up hopes for peace."

Although support for the group had been declining for years, the Spanish government turned the failure of the peace process into an opportunity to crack down on ETA. Beginning in 2007, the country's courts banned several political organizations affiliated with the group, while police have arrested dozens of suspected members. Until today, the most notable was the May detention of Francisco "Thierry" Javier López Peña, the presumed leader of the political wing of ETA, and a fellow hard-liner, who represented ETA in its talks with the Spanish government.

Spanish authorities also began to work more closely with their counterparts in France, where many ETA members have fled in recent years. This year alone, French police have arrested 31 members of the organization, according to French Interior Minister Michele Alliot-Marie.

That sort of improved police and counterintelligence work, along with growing cooperation with France, has made life harder for ETA. "There's too much pressure on the group now," says Rogelio Alonso, terrorism expert at Madrid's King Juan Carlos University. "It can't train its commandos like it used to. And when they're not trained well, they commit errors."

Mr. Sánchez-Cuenca, an ETA expert, agrees. "It's not the first time that a high-ranking member of the ETA hierarchy has been arrested, but it may be among the most critical. With all the other leaders behind bars, the person who takes Txeroki's place is going to be even less experienced, and that will leave ETA in even greater disarray."

The arrest does not mean, however, that ETA is finished. In the past, attacks have often followed detentions – an attempt by the group to prove its continued viability to both detractors and supporters alike. "It's not unlikely that they'll compensate with a terrorist attack that can animate their base," says Florencio Domínguez, editor in chief of the news agency Vasco Press and author of several books on the organization.

Prime Minister Zapatero appeared keenly aware of that possibility today as he announced the arrest. "They haven't lost their capacity to attack," he said. "They haven't lost their capacity to cause pain."