"my understanding is that it is western culture islamic terrorists (Isis) want to destroy. Something about a holy war and we either all have to die or be converted to radical Islam?"
If ISIS has been trying to convert all of us to radical Islam their campaign must be very low key indeed, because I have never heard of it.
ISIS is fighting the local governments (Syrian, Kurdish, etc) in the area where they have taken over to make a place for themselves. They were not terribly interested in us in Western Europe, but we declared war on them and we are still involved in military operations against them. So they come and attack us in retaliation.
I'm not siding with them at all. They are the scariest bunch of nutjobs on Earth. But let's be honest about what is going on and why they are attacking France, Germany, Belgium, etc.