Human events may or may not proceed according to physical laws of cause and effect (e.g. I let go of a stone, it drops). I tend to think that human events proceed according to a more complicated process in which human behaviour is in a permanent state of 'influencing each other' (mutual influence, or reflexive influence). We see this in language. When I speak to you, part of the reasons for what I say and how I say it is in my awareness of who you are. My behaviour, then is already reflexive. Then you reply, being aware of who I am, whilst being affected by what I said. Cause and effect doesn't describe this fully.
Why am I saying this?
Because the terrible events in Paris raise the spectre of cause and effect thinking. I don't believe that either saying 'ISIS caused the events' or 'the wars caused ISIS' are sufficient explanations of the processes that led to those men massacring those people. We should even be wary of saying that e.g. because a particular interpretation of Islam is a necessary condition for the ISIS mentality that this is a sufficient one. Same goes e.g. that because the wars are a necessary condition for the growth of ISIS that they are sufficient.
In other words, clearly we have lived in a time of 'reflexive influence'. Aha, say some, but this reflexiveness is not symmetrical. That's to say, one side of the question has more power than the other: so, some have implied that 'Islam' is the most powerful side of the process. Not so, say others, it's the wars and the might of the West. I agree, we have to get our relative powers sorted out in this too.
Perhaps, the most telling comment of all was Hollande saying that it was an 'act of war'. Somewhere in the reasons for him saying this, was an awareness that this deed was part of a long war in which many parties over the long history of relations between 'the West' and the 'Middle East' have justified their actions for many different reasons. But at the heart of it is the use of weapons to kill other human beings. In a terrible way, Hollande clarified that. Surely he entitled us to review this matter as a piece of terrible military history, in which for the most part the greater power was, has been and still is in the hands of the West.
War is a classic case of reflexive influence, ('arms race'), and surely now, after Hollande's words, no Western leader can pretend that if ISIS kill again it isn't part of this war being waged by both sides. In military terms, it really doesn't matter very much what ISIS say to justify what they're doing, or what the West say to justify what they're doing. We can notice it, but we shouldn't be too seduced by it.
Once we have that in our heads, we can ask 1)is either side's war justifiable? 2) millions of people have been killed in this long war, so it's urgent that we find a way to stop it going on...so does it look as if either side killing more people will stop more killing? (and yes, we have noticed that the great majority of people killed in this war are civilians).