If you know your enemy and know yourself...

“If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.”
[The Art of War]

We do NOT know our enemy!

I assume that ISIS has a strategic purpose for seemingly random bombings in European capitals.
What is it?

Simply asserting that ISIS is nihilistic, anarchical, jihadist... well, pick your favorite adjective - this isn't meaningful.
What is it that ISIS is looking to accomplish?

I don't know the answer to this question, but unless someone figures it out we are unlikely to succeed in this war.