i'm not entirely sure this was really anime - so don't flame me if i'm wrong...

the movie i'm looking for is at least 15 years old, i only saw the last part of it and here's what i remember:

there's some kind of war going on between two factions which have completely different technology - there's the dark faction with modern weaponry and the lighter side with primitive technology, e.g. instead of airplanes they have balloons, from which soldiers fire their weapons (crossbows? old guns?)

apparently the more backward ("good") side has some kind of pretection device which keeps their city save, a key or password or something, until one from their side betrays them (for money?) and opens the road for an attack

there's this one scene where two lovers are standing on a (white?) bridge in the forest between the two factions and then i think it's the woman who shoots the man and then commits suicide.

and i seem to recall a gigantic tree

any ideas?