Definition
The Block Size War was a prolonged governance conflict, roughly 2015 to 2017, over how Bitcoin should scale to handle more transactions. One camp, often called "big blockers," wanted to raise the 1 MB block size limit through a hard fork to increase on-chain capacity directly. The other camp, the "small blockers," favored keeping blocks small to preserve the ability of ordinary users to run full nodes on modest hardware, scaling instead through optimizations and second layers.
SegWit, UASF, and the New York Agreement
Segregated Witness (SegWit) was a soft-fork upgrade that effectively increased capacity while fixing transaction malleability. Miner signaling for SegWit stalled near 30%, creating a deadlock. In response, BIP148, a User Activated Soft Fork (UASF) scheduled for August 1, 2017, proposed that economic full nodes reject blocks not signaling SegWit, forcing the issue from the user side. Separately, a group of companies and mining pools signed the New York Agreement, also called SegWit2x, pledging to activate SegWit and then hard fork to 2 MB blocks.
Outcome and significance
SegWit activated in August 2017. The second part of the New York Agreement, the 2 MB hard fork, was abandoned in November 2017 after sustained opposition under the "NO2X" banner. The episode established an important precedent: users running their own nodes, not miners or large businesses alone, hold decisive influence over Bitcoin's rules.
For related consensus topics, see the Bitcoin whitepaper, which framed nodes as the arbiters of the valid chain.
In Simple Terms
The Block Size War was a prolonged governance conflict, roughly 2015 to 2017, over how Bitcoin should scale to handle more transactions. One camp, often…