On August 9, 1974, Nixon signed the resignation letter making him the first, and as of now, only, president to resign from the White House, leaving Gerald Ford to take his place as president.
His resignation came in the wake of the Watergate scandal, a burglary that took place at the headquarters of the Democratic National Committee (DNC) at the Watergate Complex in Washington DC. The burglary took place June 17, 1972, and became one of the most infamous political scandals when it became apparent that the Nixon administration was involved.
Frank Wills, a security guard, found clues that former FBI and CIA agents broke into the Watergate Complex before the election, tapping phone lines, and stealing papers.
On August 5, 1974, the "smoking gun" tape was released, where Nixon and his chief of staff are heard discussing the break-in. This destroyed Nixon politically.
Nixon chose to resign to avoid being impeached for his crimes.
Later, Ford would issue a full and unconditional pardon of Nixon, many attributing this to his loss of the election of 1976.