On Bitcoin’s Fee-Based Security Model — Part 2: Security vs. What?

Why does Bitcoin need a high hash rate? What actually goes wrong within the network if it’s not high enough?

← Part 1: Beware The Turkey Fallacy

You can’t talk about security — fee-based or otherwise — and how much security is enough without first defining what is being secured against. So what is the actual attack that’s being prevented by all that resource-intensive mining distributed around the world?