If you were
already at war with the MPP-member(?) but had not yet triggered their MPP-allies to DoW you, your "passive invasion" of the MPP-member's territory might have been enough?
Or possibly it's actually a combination of separate checks, with an 'AND' function between them, e.g. "Has units in MPP-member's territory" AND "Initiates an attack (or bombardment, including auto-bombardment) on (any) enemy unit"? So if you had units on their turf (due to a border-expansion)
and e.g. (auto?)bombed a unit, or sank a ship in your/neutral territory, that would still count as an MPP trigger.
There's certainly some non-intuitive/ wonky AND-checks for other functions, e.g. for buildings, the "Required resource must be in town radius" criterion is independent of the "Required resource = X" criterion.
Real-life modding example: To nerf the Horseman-/Knight-/Cav-rush tactic, I was/am trying to make a mod which requires Horse-based units to be "bred" (i.e. autoproduced) and then upgraded, rather than being buildable -- so I modded in a cheap "Paddock" improvement that "Requires Horses (in town radius)", and is required to build further Horse-breeding improvements.
But in-game -- and contrary to my expectation -- I found that if I controlled two Horse-tiles, one roaded but the other not, provided that the town near the
unhooked Horse-tile had a road all the way back to the hooked one, I was able to build a Paddock next to
both Horse-tiles.Wot is this I don't even