The hard fork is complete. Just before 6:30 a.m. PST, the network reached block 2463000 to execute a successful hard fork.
Yay successful #ethereum hardfork.— JΞFF (@jeffehh) October 18, 2016
#ethereum hard fork is live! 🍴— Christian Lundkvist (@ChrisLundkvist) October 18, 2016
Any decision to create a hard fork or protocol change is based on the need to fix a security flaw or possibly upgrade the system. Back in July, the reason to hard fork was to return stolen funds. This was a controversial decision resulting in the birth of Ethereum Classic. The process will move all the previous blocks onto a new blockchain, leaving the old chain invalid.
The Ethereum network was in dire need of a hard fork after being targeted by DoS (denial-of-service) attacks for almost a month. The attacks delayed transactions and activity through the network causing gas prices to rise.
The call for the protocol change came late Friday afternoon when the Ethereum Foundation announced the hard fork would occur on block 2457000 in order to stabilize the gas markets. The Foundation knew this needed to be successful. Therefore, they changed the block from 2457000 to 2463000 in order to conduct more security testing.