Secret operation organized at high levels of the Reagan administration to sell arms to Iran and funnel the proceeds to the Contras, right-wing paramilitaries undertaking terror campaigns in communist-governed Nicaragua.
Both sides of the operation were blatantly illegal. Iran was subject to an arms embargo following the storming of the American embassy in Tehran, and Congress had between 1982-1984 passed laws against funding the Contras (which Reagan had started doing in 1981).
Some weapons were sent to Iran via Israel to avoid the embargo. Much of the redirection of funds to the Contras was facilitated by the USA and Taiwanese branches of the World Anti-Communist League.
The operation remained secret until it was leaked by Islamic Revolutionary Guard official Mehdi Hashemi. Hashemi was arrested within a month (ostensibly for different crimes), tortured, and executed.
12 people were indicted in the USA and 6 convicted, though only one, CIA officer Thomas Clines, ever served time in prison. The rest were granted immunity in exchange for cooperating with the investigation or were pardoned by Reagan's vice-president and successor, George H.W. Bush.