What was the FBI’s plan when they sent a letter to Martin Luther King Jr instructing him to commit suicide? Did they honestly believe he’d do it.

It's not every day that I get to answer two questions, one about Israel and the other about MLK, but it turns out I have the sources on this at hand.

The FBI's main goal was to discredit and shame him, something they had been trying to do for some time. J. Edgar Hoover, head of the FBI, had sought to discredit King for years with investigations that crossed the line into illegal, but failed to find a way to discredit King. The FBI's head of spying operations, William Sullivan, wrote to Hoover just two days after the March on Washington that:

We must mark [King] now, if we have not done so before, as the most dangerous Negro of the future in this nation from the standpoint of communism, the Negro, and national security...

Sullivan was, apparently, the one who wrote the letter to King, seeking to try and make accusations of various indecencies he supposedly had dirt on. Writing as an angry African American, the letter contained accusations that King was a "great liability to all of us Negroes", and saying that King was "finished". The threat near the end, that King had "just 34 days" in which to commit suicide before his "filthy, abnormal fraudulent self is bared to the nation", was accompanied by audio recordings made by spying on King's hotel rooms, and were recordings of King talking to women other than his wife.

The FBI apparently did believe that the threat would lead King to commit suicide, but King rightly suspected that the threat was coming from the FBI, and obviously did not commit suicide. While Hoover loyalists always claimed that Sullivan acted on his own, the complexity of the package's contents, the traces of it in FBI files, and the "climate of fury" (according to Taylor Branch's Pillar of Fire) all suggest it was an institutional operation, not something Sullivan did on his own.

King's reaction was notable. Coretta, his wife, read it first, and King himself listened to the tape three times before rushing in his inner circle, for whom he played the tape numerous times. Ralph Abernathy, Andrew Young, Joseph Lowery, and Bernard Lee "played the tape over and over", looking for clues in the wrapping and postmark for where it originated. They all agreed: they believed it came from the FBI. And the FBI knew it had an effect on him. King was recorded saying, for example, that "they are out to break me", and he kept word of the package quiet.

King's friends launched into the fray. They confronted the FBI, discussing malicious leaks and accusing the FBI of malintent. By all accounts, the FBI denied involvement, but still hoped King would be driven to suicide. The House committee, also known as the Stokes Committee, also suggested that it was sent as an institutional operation run by the FBI, but could not confirm it.