Because the bitcoin blockchain is getting larger and larger in size people often don't use the original bitcoin client that fully downloads the blockchain to your computer. As I understood the other wallets connect to an online server that has the blockchain on it to be able to connect with the network. Would it be possible for an attacker to upload a different blockchain history to that server so that possibly millions of users would send transactions to a wrong chain or have wrong balances displayed? Has this happened in the past or is this not possible?