What Is a ‘Hard Fork’ and How Can It Be Used as a Defense against a Persistent 51% Attacker?

A hard fork is a permanent, non-backward-compatible change to the blockchain protocol that requires all nodes to upgrade. As a defense, the community can coordinate a hard fork to change the Proof-of-Work mining algorithm (e.g. from SHA-256 to a different algorithm).

This action instantly renders the attacker's specialized mining hardware (ASICs) obsolete, effectively resetting the hashrate distribution and neutralizing the attack.