What hard fork created Ethereum Classic?

The hard fork that created Ethereum Classic was the DAO fork.

In June 2016, a vulnerability in The DAO’s smart contract allowed an attacker to divert roughly 3.6 million ether. The incident created a crisis because the stolen funds were temporarily locked by the contract’s withdrawal rules, giving the Ethereum community time to debate a response.

On July 20, 2016, Ethereum implemented a hard fork at block 1,920,000. The altered chain preserved the Ethereum name and attempted to return the affected funds. Participants who rejected the intervention continued the original, unaltered chain, which became Ethereum Classic.

A common mix-up is confusing the DAO fork with later Ethereum upgrades such as Byzantium, Constantinople, or Istanbul. Those were protocol upgrades, whereas the DAO fork was the controversial chain split that produced two networks.