Documentation.
This.
It's a link to a translation of the original documentation. And it's an ORDER to use the weapons being provided in the event of an invasion.
Regarding Nixon and negotiating. I don't think it's too much of a leap, though it is a judgement call, to believe that Nixon was less inclined to subtleties and negotiating than Kennedy.
I just think Nixon was more likely to take things personally and strike. I don't remember Kennedy's "enemy list", though he did get irritated and cut off access to the White House once in a while.
(Read Ben Bradlee's "Conversations with Kennedy". JFK even cut off Bradlee once.
Arf