Attacker rolls first then the defender allowing the defender to choose how many dice to roll. The idea being that the defender has the advantage. In GOT risk it also allows you to choose whether to use certain cards etc.
At least that's the way we always played. The latest version of the rules for the classic game state that you should roll at the same time though.
Before rolling, both you and your opponent must announce the number
of dice you intend to roll, and you both must roll at the same time.