As always after a major terrorist attack on the West, the right question to ask after the slaughter in Paris is: What were the strategic aims behind the attack? This requires getting your head around the concept that terrorists have rational strategies, but once you have done that the motives behind the attacks are easy to figure out. It also becomes clear that the motives have changed.
The 9/11 attacks on the United States in 2001 followed the classical terrorist strategy of trying to trick the target government into overreacting in ways that ultimately serve the terrorists' interests. Al-Qaida's goal was to sucker the U.S. into invading Muslim countries.
Al-Qaida was a revolutionary organization whose purpose was to overthrow existing Arab governments and take power in those countries, which it would then reshape in accord with its extreme Islamist ideology. The trouble was that Islamist movements were not doing very well in building mass support in the Arab world, and you need mass support if you want to make a revolution.