Mutual attacks - where do they fight?

[Firebreath]

Thanks for helping me clear up the following:

If two armies attack each other - they (presumably) can't fight in the middle, but rather on the land of one, or on the land of the other - so how is this decided?

If one of the armies is a flying only army, does this affect things? Does the flying army engage the other attacking army, or does it fly over and attack the province where the other army came from, leaving the other army to attack your province? And: what if the flying army is flying over an attacking enemy army - onto a province beyond the attacking enemy army, does it ignore the attacking enemy and fly straight over onto its destination?

Finally, it seems that ocassionaly it is possible to send very small armies against a much bigger army - and so block them (stopping them from attacking you by attacking them instead) - has anyone else experienced this?