Use of the special power "Diplomat" against another player not yet entered the game in Smallworld
I found an answer to this question in an answer to a similar question:

Q: Is a race with the Diplomat power allowed to nominate a player, who has just declined or who has not yet had a turn (and therefore has no active race) as an ally?
A: Yes indeed, the only restriction in the choice of an ally is that you cannot choose a player if you just attacked his active race.