How Does the 51% Attack Relate to the Ability to Execute a Blockchain Re-Org?

A 51% attack occurs when a single entity or coordinated group controls more than half (51%) of the network's total mining hash rate (Proof-of-Work) or staking power (Proof-of-Stake). This majority control allows the attacker to mine blocks faster than the rest of the network combined.

By mining blocks privately, they can create a longer chain, thus enabling them to force a re-org and rewrite history, which is the core mechanism for a successful attack.