Watergate was the first time the FBI (and CIA?) engineered the overthrow of an elected President.
It was like a new weapon which was never seen before, so of course, the targets were unprepared.
It proved, in spades, that a president is not his own boss. He must absolutely make sure his people control the DOJ and have power within the Intelligence Community.
In that light, I don’t think everyone realizes how precarious DJT’s position was with Sessions as his AG.
Second.
No I don’t think that was the first time, I think JFK’s killing was one also.