How Do ‘Reorgs’ (Reorganizations) in a Blockchain Relate to the Success of a 51% Attack?

A reorganization (reorg) is when a node switches from one chain tip to a longer, valid chain, causing a temporary rollback of transactions on the shorter chain. A 51% attacker exploits this by secretly mining a longer chain where their fraudulent transaction (the double-spend) is included, but the original, legitimate transaction is not.

Once the private chain is longer than the public one, the network adopts the attacker's chain, making the double-spend successful.