In obtaining power stewardship clearly outranks diplomacy.
but at the end of the day what decides victory=TRUE|FALSE is your power. This truly is an elegant, superb game, I think the best game I ever played and I can see a nice chunk of my life being invested (or wasted according to my gf) into this game having said that in the simplistic high-level look, this game is not much different than many others as it is all about power true that diplomacy helps you etc. In the light of what you've posted above I think it is indeed important but I would still place it second after stewardship as stewardship equals more holdings directly under your control (demesne or demense I will figure one day) which equals to more power (=troops + gold). As a matter of fact I roleplayed a good 'player' and searched for it in the manual and unsurprisingly it is NOT there as all you can see is 'diplomacy interface'. This command adds the specified amount of diplomacy to the character with the specified ID. I wasn't aware what Diplomacy actually helped apart from the obvious (=other rulers are more likely to agree/accept your proposal).
