What Role Do 51% Attacks Play in Enabling Double-Spending?

A 51% attack occurs when a single entity or group gains control of more than 50% of the network's total hashing power. With this majority, the attacker can mine a private, longer chain that includes a double-spend transaction.

They can spend coins on the public chain and then use their private chain to invalidate that transaction, effectively getting their coins back. The network's "longest chain rule" forces other nodes to accept the attacker's chain, enabling a successful, large-scale double-spend.