There was a hack on 17th June 2016 when someone found a loophole in the DOA and siphoned away one-third of the DAO’s funds around $50 million dollars. There’s someone that is the most certain of them all and that is the hack happened because of an issue in the DAO and not because of any issues related to Ethereum. You should know one more thing and that is, Ethereum runs in the background whereas DAO runs on it. So you can. simply blame Ethereum for the DAO hack. The co-founder of Ethereum, Gavin Wood said, it’s like saying “the internet is broken” every time a website goes down.
After this incident, Ethereum planned to implement a soft fork. When the entire community was on board with this plan then a problem surfaced. And this problem brought the entire community to another difficulty and that was initiating the process of implementing a soft fork would simply result in a DoS(“Denial Of Service”) attack.
For the record, the only way through which miners are protected from DoS attacks in Ethereum is the reward “Gas” that manages all mining activities in the ecosystem. The miners of Ethereum network execute the computations and even if they fail to complete the computation, they get a Gas score. The gas score is equivalent to a number of computations that they have done. If the computation is a time consuming and difficult then the Gas reward is more and due to this the attacker have to spend a lot of their own money to make these attacks happen. And once the soft fork is implemented, attackers can easily run around the system and he can flood the network with transactions that interact with the DAO. This will make the miners do endless computations for little or no Gas price. The attacker might also trick the miners into solving a malicious computation and set a high Gas price for it.
And this is the reason why Ethereum community has to undergo a Hard Fork. The hard fork in Ethereum made it possible to separate the Blockchain into two blockchains. The new chain was named as Ethereum and the older Blockchain was named as Ethereum Classic. The people who opposed to the hard fork stuck with the original chain.