I think there is a marked difference between ISIS affiliated and ISIS supported. Any group can claim affiliation/solidarity with ISIS. But I really do recommend reading the Atlantic piece, it's the only deep dive I've ever seen written that explains some seemingly inexplicable behavior.

That said, I agree this is likely retaliation, I'm just not sure how big the "enemy" we're fighting in Belgium is. Today's attack took a minimum of what, 3 people?