...confuses me and I´ve repetedly made the "wrong" choice as a result. Maybee it´s the fact that english isn´t my native language.

This is what it says:

"War declared on ally:
On: XXXX
Ally: ZZZZ"

"On" is aparantly the agressor so "on" should be "by" instead. By saying "on" you get the notion that it´s the "ally" that´s the agressor. Or am I completely dumb?!