Re: How to make an alliy turn on me?
mmm will be difficult, first you have to cancel alliance and trade rights with them (through diplomacy), but if IIRC this will bring down you reputation anyway, than if the relations are already not so good and you own some of their settlements for victory conditions they will quite surely attack you.
Will Gondor remain on your side? good question 
It will depend on relations between you and them and Rohan and them.
Hope it helps
Last edited by Flinn; February 14, 2013 at 10:49 AM.