Reentrancy attacks, such as the one that compromised The DAO, exploit vulnerabilities in Solidity code, particularly through fallback functions and the order of operations within smart contracts. The attack on The DAO in 2016, which involved a "blackhat" hacker draining nearly $150 million worth of Ether, highlighted significant security flaws in early Ethereum smart contract development. This breach led to intense debates within the Ethereum community about immutability versus intervention, ultimately resulting in a hard fork that split the blockchain into Ethereum and Ethereum Classic. The reentrancy exploit works by repeatedly calling a vulnerable function before previous executions finish, allowing the hacker to withdraw funds without updating their balance. The DAO hack and the response to it marked a pivotal moment in blockchain history, illustrating both the potential and risks of decentralized technologies. To prevent such attacks, developers can implement strategies like reordering operations to update balances before executing transactions or using reentrancy guards to block multiple concurrent invocations of a function.