The question is, why are they attacking western civilians? There could be a few different reasons, for different kinds of attacks.
When they are striking against groups of tourists in the Arab world, as on that beach in Tunisia or in Egypt, their idea would be to hurt the income of those countries from tourism (a very big thing for countries like Egypt) and to make the local government come across as being in hock to the West. If few people want to go to Egypt because of spectacular terrorist acts, then the local rulers will have a tougher time. Now I don't think the Muslim Brotherhood in Egypt are actually any allies of IS, though both movements want an islamic society in sme sense, but IS wants to polarize things so that some of the people who supported the MB might join the ranks of IS instead.
With those beheadings and killings on camera of captive western hostages, it's for propaganda value I figure. It's a way of boasting "we're standing up to the west, to the infidels! this is how powerless they are to stop us!"
So, a few western countries have attacked from the air, the US has been using drone strikes for some time too, and IS replied with the cowardly raid in Paris. That's a different kind, and it would bring together a few different reasons, I figure:
- 1. They may have wanted to put presssure on countries in the west to stop attacking them, by "hitting where it hurts" and hoping that the people of France and other countries won't stand up to this. They certainly see the west as a culture of weak people who can be easily pushed down or, in the future, enslaved and controlled. This doesn't have anything to do with how the big majority of ordinary muslims, educated muslims see history, but it's part of the outlook of these ratbags.
- 2. Vengeance. Visible revenge for drone strikes and air strikes (though if the top people of IS have any sense they'll realize that raids like the one in Paris, or in America, will just lead to a barrage of new air strikes and more determination to hit back at them)
- 3. Inspiring spin-off terrorist cells and local movements in the west. Nobody's been talking much about whether IS might want to get involved with any kind of local groups that are not overtly violent, not just into throwing bombs and the like, but which would help disseminate their ideas anyway. They do have some links to radical mosques and networks in western Europe...it's at least conceivable that they'd see it as an aim to become a steady presence in some ghetto suburbs and exercise some control of daily life there, creating "no-go zones". That way they would be taking the war into the enemy's country. And no doubt they would want to grow local terrorist cells and outfits in the west. The band that struck in Paris was largely made up of Belgian and French-based Arabs, though the ringleader was an IS-trained "professional".
It's a bit surprising they have not (to my knowledge) tried to attack in Israel yet, it would fit all three of those reasons (it's certainly an antisemitic movement and there's no doubt they think muslims have been backstabbed by Jews, and by Israel - so they already see the country as a prime enemy). It could be the next step. :