What Is the Primary Defense Mechanism against a 51% Attack on a PoW Network?

The primary defense is the sheer economic cost required to acquire and maintain the necessary computing power (51% of the hash rate) to execute a sustained attack. The massive initial capital expenditure for specialized hardware and the continuous, exorbitant energy cost create a significant economic disincentive.

Furthermore, if an attack were successful, the resulting loss of trust would likely crash the coin's price, making the attack financially self-defeating for the attacker.