Overview
The Blocksize War was the most consequential governance crisis in Bitcoin's history, spanning roughly from 2015 to 2017. At its core was a deceptively simple question: should Bitcoin's 1 MB block size limit be raised to accommodate more transactions? Beneath this technical question lay a fundamental disagreement about what Bitcoin is, who controls it, and how protocol changes should be made. The conflict pitted influential companies, miners, and developers against each other and ultimately tested whether any coalition of powerful actors could force a change on the network against the wishes of node-running users.
The war ended with the activation of SegWit as a soft fork in August 2017 and the creation of Bitcoin Cash (BCH) as a hard fork the same month. The outcome established a lasting precedent: Bitcoin's rules are enforced by the decentralized network of full nodes, not by miners, corporations, or even core developers.
Background and Key Events
The debate had roots going back to 2010, when Satoshi Nakamoto quietly added the 1 MB block size limit as a spam-prevention measure. As Bitcoin grew, some argued this limit would eventually choke the network. Others countered that increasing it would raise the cost of running full nodes, centralizing validation power in the hands of well-resourced entities.
Timeline of the Blocksize War:
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2015 Bitcoin XT — First hard fork attempt (8 MB blocks)
Failed to gain majority hash power or node adoption
2016 Bitcoin Classic — Proposed 2 MB hard fork
Also failed to achieve consensus
2016 Hong Kong Agreement — Miners and devs compromise
SegWit + 2 MB hard fork pledge (never fulfilled)
2017 Bitcoin Unlimited — Variable block size proposal
Gained miner support but suffered software bugs
2017 New York Agreement (NYA/SegWit2x) — 58 companies
Planned SegWit + 2 MB hard fork within 6 months
Aug '17 BIP148 UASF — User-activated soft fork threat
Node operators forced SegWit activation
Aug '17 SegWit activates on Bitcoin (block 481,824)
Aug '17 Bitcoin Cash hard forks (8 MB blocks)
Nov '17 SegWit2x hard fork cancelled — insufficient support
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The Two Sides
Big blockers argued that Bitcoin needed to scale on-chain by increasing the block size to keep transaction fees low and enable everyday payments. Prominent supporters included Roger Ver, Gavin Andresen, and mining companies like Bitmain. They believed Bitcoin's value proposition depended on cheap, fast transactions for everyone.
Small blockers argued that keeping blocks small was essential to preserve decentralization by making it feasible for ordinary users to run full nodes. They favored scaling through layered solutions like the Lightning Network and efficiency improvements like SegWit. Key figures included Adam Back, the Bitcoin Core development team, and grassroots node operators.
The UASF and Resolution
The decisive moment came with BIP148, the User Activated Soft Fork (UASF). Rather than waiting for miner signaling, node operators announced they would reject blocks that did not signal for SegWit after August 1, 2017. This forced miners to choose: signal for SegWit or risk mining on a chain that a significant portion of the economic network would reject. Miners capitulated, and SegWit locked in.
The faction that wanted larger blocks proceeded with Bitcoin Cash, which launched on August 1, 2017, with an 8 MB block size limit. The subsequent SegWit2x hard fork, which had backing from major companies through the New York Agreement, was cancelled in November 2017 when it became clear it lacked sufficient community support.
Legacy
The Blocksize War established several principles that continue to define Bitcoin governance. Users running full nodes are the ultimate arbiters of protocol rules. Hash power alone cannot dictate consensus changes. And any change to Bitcoin's rules requires overwhelming, broad-based agreement across the entire ecosystem. Jonathan Bier's book The Blocksize War provides the definitive account of these events.
Related Concepts
- Hard Fork — The mechanism big blockers proposed for increasing the block size
- SegWit — The soft fork that resolved the immediate scaling pressure
- Full Node — Running nodes proved decisive in enforcing user-chosen rules
- The Blocksize War — Jonathan Bier's comprehensive book on the conflict
- Adam Back — Hashcash inventor and prominent small-block advocate