It’s well known that former President John F. Kennedy cheated on his wife Jackie Kennedy many times, but one affair stood out among all the rest, and that was the one he had with the Hollywood star Marilyn Monroe. A recent biography revealed that Monroe once called Jackie to say that Kennedy had promised to marry her, and the First Lady had the perfect response.

“Marilyn, you’ll marry Jack, that’s great,” Jackie replied, according to Vanity Fair. “And you’ll move into the White House, and you’ll assume the responsibilities of the first lady. And I’ll move out, and you’ll have all the problems.”

Jackie’s personal letters that were obtained by The Boston News show that she had known Kennedy would cheat on her before she married him.

“He’s like my father in a way,” she wrote. “Loves the chase and is bored with the conquest. And once married needs proof he’s still attractive, so flirts with other women and resents you.”

“I think he was as much in love with me as he could be with anyone,” Jackie added. “And now maybe he will want to get married because a Senator needs a wife. But if he ever does ask me to marry him, it will be for rather practical reasons – because his career is this driving thing with him.”

Through all of Kennedy’s affairs, Jackie continued to love him because “he always came home to her.” The affair with Monroe, however, was different.

“She viewed his dalliance with Marilyn differently from the others for a number of reasons,” biographer Charles Cassilo wrote. “It wasn’t that Jackie felt threatened by Marilyn- she was too assured of her importance in Kennedy’s life- but she understood the fascination that surrounded the blond star.”

“More than anything, Jackie understood that her family would be publicly disgraced if it somehow came out that her husband was having an affair with Marilyn. Plus, Jackie felt a great deal of empathy for her,” he added. “She knew that Marilyn was a deeply sensitive, troubled woman. ‘This one is different, Jack. Have some pity on her,” she warned. ‘I want you to leave Marilyn alone.’”

Monroe died of a drug overdose in 1962, one year before Kennedy was assassinated. Jackie would live until 1994, when she died of cancer at the age of 65.