I'm describing an attack. An attack happens when the attacker doesn't play by the rules, while the rest tries to. Here an attacker controlling 51% of mining po

I'm describing an attack. An attack happens when the attacker doesn't play by the rules, while the rest tries to. Here an attacker controlling 51% of mining power decides to not play by the rules; we are considering what that means for everybody else. For Bitcoin it means they can block transactions forever, and they can double-spend. Obviously if the attacker plays by the rules there is no attack.