This churn at the top is good news. Each arrest forces ETA to reorganise, leaving it less time to plan and to plant bombs. Mr Martitegi and seven others arrested with him seem to have been planning a car-bomb attack in early May. That would have given a bloody welcome to Patxi López, a Socialist poised to take over as the Basque region's first non-nationalist premier since 1980.

The serial arrests of ETA's military chiefs may be even better news in the long term. The four captured over the past year apparently belonged to a hardline faction that pushed for an end to ETA's ceasefire two years ago. Now there are signs that the peaceniks have regained control. Josu Urrutikoetxea, a veteran who took part in talks during the ceasefire, is said to be back near the top. Arnaldo Otegi, who has served time in jail for terrorist offences, has reappeared as a spokesman for ETA's political arm. He is said to want a negotiated end to four decades of violence.

Yet the Madrid government of José Luis Rodríguez Zapatero may not be in a mood to negotiate. ETA made a fool of Mr Zaptero in late 2006, when it exploded a car bomb at Madrid's Barajas airport, killing two people, during its most recent ceasefire. The interior minister, Alfredo Pérez Rubalcaba, noted that this was the third time that a Spanish government had been burnt after talking to ETA. There will not be a fourth, he declared after the latest arrests.

© The Economist Newspaper Ltd. 2009