The 2016 event that split Ethereum into Ethereum and Ethereum Classic was the DAO fork.
The split followed the attack on The DAO, an Ethereum-based investment project. In June 2016, an attacker exploited a vulnerability in The DAO’s smart-contract code and moved a large amount of ether into a child DAO. The funds were subject to a withdrawal delay, giving the community time to debate a response.
Ethereum developers and participants chose a controversial hard fork in July 2016 that redirected the affected funds. The chain that implemented the change kept the Ethereum name. Participants who rejected the intervention continued the original chain, which became Ethereum Classic.
The event demonstrated that blockchains can face social and governance decisions even when their transaction rules are automated. It also established an important distinction: “immutable” describes protocol history ideals, but communities can coordinate software changes that produce a new chain.