What Is the Primary Defense Mechanism against 51 Percent Attacks for PoW Coins?

The primary defense mechanism is achieving a sufficiently large and decentralized hashrate. A high total hashrate makes the attack prohibitively expensive and logistically difficult for any single entity to sustain.

Decentralization ensures that no single geographical region or mining pool can easily amass the required 51 percent. Additionally, some coins implement checkpointing or delayed finality mechanisms to increase the cost of chain reorganization.

Strong community vigilance and immediate reaction to unusual mining activity are also crucial non-technical defenses.