Oh for the love of dog. Eric Trump's tweet on Dec. 31 2019.

[FBaggins]

11. Anything at all

The president says he wants to strike back... tweets his "threat, not a warning" nonsense and tells the situation room to come up with options ASAP.

At that point, Jr knows that we're going to do something... but that doesn't mean that he knows anything. It's possible that nobody knew much of anything at that point. Situation room (likely through a call to a golf course) gives him five options and he says "Ooh... I like that one" - but that might have been mere hours before the strike.