No one can force a President to resign if he doesn't want to; however, past history has shown a President, Nixon, had only two alternatives; either remain and go through the impeachment process, which had already begun, and by which Nixon was going to be convicted and removed from office, or resign. Nixon choose resignation, so he was, in effect, 'forced', since there were no other viable options available. You may not want to call it forced, but there really is no other reasonable description...
Incidentally, the Nixon Presidential Library, until fairly recently, did not acknowledge the resignation of Nixon nor the Watergate sandal that ousted him. In the Library's exhibit of the chronological history of Nixon's life and political career, the exhibit basically used to get to the point of the resignation and just commented Nixon 'left' office on 9 Aug 1974; no why or how; Nixon just 'left'. The exhibit then when on to describe Nixon's life as a private citizen. In recent years, pressured by historians and others, the Library has finally changed the exhibit to better reflect what actually occurred leading up to Nixon's resignation...
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