How Did Ethereum’s Transition to Proof-of-Stake (PoS) Fundamentally Change Its Security Model?

Ethereum moved from a Proof-of-Work (PoW) security model, which relied on miners and computational power, to a Proof-of-Stake (PoS) model, which relies on validators staking their ETH. Security is now guaranteed by the economic cost of attacking the network.

A malicious validator can have their staked ETH "slashed," an economic penalty that makes attacking the network prohibitively expensive, replacing the energy cost of PoW with a capital cost.