Complex sentence: Andrew Johnson in 1868 and Bill Clinton in 1998--were both impeached; both were also acquitted from all charges following a Senate trial, meaning that the impeachment process against Nixon is the only one that has led to the departure from office of its target.
Split into simpler sentences: Andrew Johnson in 1868 and Bill Clinton in 1998--were both impeached; both were also acquitted from all charges following a Senate trial. The impeachment process against Nixon is the only one resulting in the resignation from office of its target.