>>60547953A 51% attacker can do essentially two things
1. Reorg the chain, basically changing the past for some purpose (censorship, double spending, theft (every block reward can be claimed by the 51% attacker, stealing it from the other miners if they haven't sold yet)). This has the caveat that the miner must duplicate the work of older blocks, so the risk of a reorg falls exponentially as more and more blocks are mined after it, 51% attack or not.
2. The same power as above but for future transactions, e.g. censoring transactions, claiming all block rewards unilaterally killing miner incentives, etc. This could be used for maximal damage by denying all transactions, only mining empty blocks which you then dump on some exchange, effectively killing the network.
This is because, unlike what maxis like
>>60548721 believe, the consensus mechanism of Bitcoin is that the chain with the most proof of work is the most valid chain, so the moment any one actor has more hashpower than every other actor, they control the full consensus of Bitcoin for the future and to some degree the past. Unlike proof of stake networks, which can simply censor the attacker by removing/slashing his stake, proof of work networks have no vehicle to censor the attacker outside of changing the hash algorithm. An attacker has financial incentive to destroy Bitcoin like this if he is fundamentally short Bitcoin, whether they be a real short position, a long position in a competing network that would survive BTC's death, or as mentioned a foreign nation using it as an act of war.
