What led to the release of President Nixon's White House tapes to the Senate?

There was a break-in, FBI Ordered Tapes.

Explanation:

There was a break-in at the Water Gate hotels, where the Democratic Committee Headquarters were. After some investigation, there was new information found that members of Nixon's administration gave the approval for illegal activites. The people of the United States wanted Nixon impeached, so the investigation of the Water Gate continued. The Supreme Court then ordered him to hand over the tapes - in which Nixon told the FBI to end the investigation. He was the first president to resign in U. S. history, and was later pardoned of his crimes by Gerald Ford.

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