If I'm defending with units with counterstrike and they manage to kill everyone with counterstrike, is combat still considered to have taken place?

Yes, I believe so.

RM

I think so as well since damage in ballte is assigned and applies at the same time.

//Baersson

Kind of irrelevant - counterstrike damage happens at declaration of defenders, not when combat damage is assigned.

But yeah, I'm sure that even if all the attackers leave combat for some reason (returned to hand, leave the zone, etc etc etc) combat is still considered to have happened.