Early in the aftermath, Prime Minister José María Aznar pins responsibility for the bombings on Basque separatist group ETA, a move that ultimately costs his party the next presidential election and draws fire as demonstrative of the ineptness of Aznar's administration.
Because the terrorists responsible weren't ETA. The terrorists responsible were Islamic, inspired by (though not necessarily all members of) al-Qaeda and similar fundamentalist cells. But why would Islamic
Most people you ask would say the attacks were due to Spain's involvement in the Second Gulf War, then blossoming as a full-blown insurgency. At the time, about 1.300 Spanish members of the Plus Ultra Brigade, an international conglomerate of 2.500 troops from Spain, the Dominican Republic, El Salvador, Honduras, and Nicaragua, had their boots on the ground in Iraq. But the role of the Spanish Army wasn't military, but humanitarian, spending most of its time disseminating aid to the surrounding population. With the exception of a few isolated skirmishes, the Brigade's guns remained silent, bound by severely restricting Rules of Engagement. So why, then, would al-Qaeda choose to strike at a country of no particular military importance in the war? What did they hope to achieve?
My theory is that the attack on Spain was symbolic, more psychological than tactical. Spain's involvement in the war was already limited and the Spanish population at best lukewarm in its support. It would not take much to prompt a withdrawal, which could then be seen as a victory. When Spain retracted its troops in the aftermath of the 11-M bombings, one must realize the important boon this must have been for al-Qaeda and the insurgency's recruiting: they had brought a nation to its knees and forced it to surrender before them. If they could do that to Spain, then they could do it to other countries, and maybe even the US. It was a proof of concept that the fight they were carrying out could be won: a morale booster and political statement.