Maybe it isn't as obvious to others, and if not then sorry if my guess is a "spoiler" ... . but
logically it makes sense.
First you have Alexandria, where Rick just watched two of his own get brutally murdered.... Sooner or later they will want their revenge.
Next you have The Hilltop, who have already tried to hire Rick and his group to take out Negan.
Lastly, you have The kingdom feeding the Dead to the Pigs as a FU to the Saviors and Negan..
all 3 alone know they have no chance against Negan and his army.. But together............
So yea, in my mind it seems pretty obvious where this is heading..