Conversation so far:
who was the last president to be impeached
On December 18, 2019, the House of Representatives voted to impeach Donald Trump.
Was Donald Trump removed from office?
As both articles of impeachment failed to receive the required super-majority, Trump was acquitted and was not removed from office.
Did impeachment of the president ever work?
While Richard Nixon was never impeached, his impeachment remains the only successful impeachment process, as its intended effect was carried out when Nixon resigned from office.

Question: When was the resignation?
Self-contained rewrite: When was the resignation of Richard Nixon as president?