The Block Size War ran from the summer of 2015 to November 2017, and its decisive split came right after block 478,558. Up until that block, Bitcoin had one chain, one set of rules, one community, grudgingly. Starting at block 478,559, two chains existed: the original Bitcoin (which locked in SegWit eight days later, at block 479,808), and Bitcoin Cash (which rejected SegWit and chose larger blocks instead). Same history up to block 478,558, divergent histories from then on.
Both chains are still alive in 2026. Both still claim the Satoshi-era coins. The fork is the cleanest example of a chain split in Bitcoin's history, and the on-chain artifacts are striking.
The shared history
Pull the last block before the fork:
Block 478,558 was mined on August 1, 2017, at 13:16 UTC. Its median-time-past, 12:37 UTC, was the first to clear Bitcoin Cash's 12:20 UTC activation time, which is why the next block became the fork block. Up to and including this block, the chain that Bitcoin and Bitcoin Cash share is byte-identical. Every transaction, every UTXO, every coinbase reward.
The fork point
At block 478,559, the two chains diverged. The first BCH-only block on the Bitcoin Cash chain was mined later that day by ViaBTC. It wasn't accepted by Bitcoin (BTC) nodes because the block itself was 1.9 MB, nearly double the 1 MB limit BTC's consensus rules enforce. The Bitcoin Cash fork rules required the first fork block to exceed 1 MB, so legacy nodes could not follow it.
On the Bitcoin chain, block 478,559 is a normal SegWit-signaling block. Pull it:
From this block on, the two chains permanently differ. Same history before, different history after. Every BTC holder before the fork ended up holding the equivalent BCH at the moment of split (since UTXOs were shared up to block 478,558).
The "replay protection" question
Because both chains shared UTXOs at fork time, a transaction valid on one chain was, in principle, also valid on the other. Bitcoin Cash added "replay protection" by requiring a new sighash flag, ensuring BCH-signed transactions wouldn't validate on BTC and vice versa. Without that, a user spending on one chain could accidentally have the same transaction confirm on the other.
Bitcoin's chain (BTC) is what our node tracks. Querying it gives the canonical, larger-economy history. Bitcoin Cash continues on its own network, has had its own subsequent forks (BCHN vs Bitcoin ABC's BCHA, now eCash, in 2020), and represents a much smaller fraction of total cryptocurrency value in 2026.
What we can see, what we can't
Our bitcoind only follows the Bitcoin chain. Querying for any block from height 478,559 onward will return the BTC side of the fork. To see what BCH looks like at the same height, you'd need a Bitcoin Cash node, those run different rules and different software.
But we can still surface the shared history:
Same UTXOs, same coinbase rewards, same transaction hashes, on both chains up to this point. Then divergence.
Two years of war, five hours of fork
The argument leading up to August 1, 2017 was about Bitcoin's most fundamental tradeoff: should the network optimize for cheap on-chain transactions (large blocks, lower decentralization) or for self-custody and node operability (small blocks, higher decentralization)?
SegWit was the small-blocker compromise: more capacity per block via weight units, plus second-layer scaling primitives (the malleability fix enables Lightning Network). Bitcoin Cash was the large-blocker answer: just make blocks bigger.
In 2026, Bitcoin's market cap dwarfs Bitcoin Cash's. Lightning Network handles small payments off-chain. The fee market has settled. The small-blocker thesis is now the dominant one. The fork was its permanent vote.
Sources
- The Blocksize War, Jonathan Bier, March 11, 2021. The standard account of the war, dating it from August 2015 to November 2017.
- Segwit2x Final Steps (bitcoin-segwit2x mailing list, archived copy), Mike Belshe (signed with Wences Casares, Jihan Wu, Jeff Garzik, Peter Smith and Erik Voorhees), November 8, 2017. The suspension of the SegWit2x 2 MB upgrade, the war's end point.
- UAHF Technical Specification v1.6, Bitcoin ABC / Bitcoin-UAHF, July 24, 2017. The 12:20 UTC median-time-past activation time (REQ-2), the rule that the fork block must exceed 1,000,000 bytes (REQ-3), and the SIGHASH_FORKID replay protection (REQ-6-2).
- Blockchain.com Haskoin BCH API, block 478559, Blockchain.com (BCH chain data), August 1, 2017. The first Bitcoin Cash block, on a chain our node does not follow: 1,915,175 bytes, mined at 18:12:41 UTC, with a ViaBTC coinbase tag.
- Bitcoin Core, src/kernel/chainparams.cpp (consensus.SegwitHeight = 481824), Bitcoin Core developers, master branch, read October 5, 2026. Core's hard-coded SegWit activation height, 481,824 (August 24, 2017), one 2,016-block period after the 479,808 lock-in; both came after the August 1 split.
- BIP 141: Segregated Witness (Consensus layer), Eric Lombrozo, Johnson Lau, Pieter Wuille, December 21, 2015. Block weight, the malleability fix that Lightning depends on, and the BIP 9 bit 1 deployment.
- BUIP-HF Digest for replay protected signature verification across hard forks, Bitcoin ABC / Bitcoin-UAHF, July 16, 2017. SIGHASH_FORKID signatures are invalid on chains without the mechanism, so BCH transactions do not replay on BTC.
- Bitcoin ABC will support both BCHA and BCHN after the chain split (archived copy), Bitcoin ABC, November 6, 2020. The November 15, 2020 BCHN/BCHA split.
- eCash - Wealth Redefined, Bitcoin ABC, June 28, 2021. The July 1, 2021 rebrand of BCHA to eCash (XEC).
- CoinGecko simple price API (bitcoin, bitcoin-cash), CoinGecko, checked October 5, 2026. Market caps on the check date: BTC about $1.72 trillion, BCH about $6.4 billion.
- Blockchain.com Haskoin BCH API, best block, Blockchain.com (BCH chain data), checked October 5, 2026. BCH tip at height 971,665 on the check date; the chain is still being mined.
Read more: The Inflation Bug, The Mt. Gox Collapse, or The Genesis Block.