Crypto glossary
Blocksize war
The blocksize war was the 2015 to 2017 fight over raising Bitcoin's 1 MB block size limit, which ended with SegWit's activation and the Bitcoin Cash split.
As transactions grew, blocks filled up and fees rose. One camp, including several large companies and miners, wanted to raise the 1 MB limit through a hard fork so that Bitcoin could process more payments on-chain. Another, including most Bitcoin Core developers, argued that bigger blocks would make running a full node harder and centralise the network, and preferred SegWit plus off-chain scaling like Lightning.
Competing clients such as Bitcoin XT, Bitcoin Classic and Bitcoin Unlimited tried to win miner support. In 2017 a group of companies signed the New York Agreement for SegWit plus a later 2 MB hard fork, while users pushed a user-activated soft fork to force SegWit. SegWit activated in August 2017; big-block supporters had already launched Bitcoin Cash on 1 August, and the planned SegWit2x fork was cancelled in November.
The episode established that users running nodes, not only miners or companies, decide Bitcoin's rules, and it left lasting distrust between factions. It also shaped Bitcoin's conservative approach to protocol changes, visible in later debates over covenants and data limits.
Example
In 2017, a user running a node that enforced the user-activated soft fork rejected any block that did not signal support for SegWit, adding pressure on miners to activate it.