What Is the Significance of the “Longest Chain Rule” in Executing a Double-Spend Attack?

The "longest chain rule," or Nakamoto Consensus, dictates that nodes in a Proof-of-Work network must always consider the chain with the most cumulative work (the longest chain) as the valid one. An attacker leverages this rule by secretly mining a chain that includes a conflicting transaction (the reversal of the initial spend).

Once their secret chain surpasses the length of the public chain, they release it. Honest nodes automatically switch to the attacker's longer chain, effectively nullifying the original, spent transaction and completing the double-spend.