Problem: you can't choose which army participate in a fight when there is more than 3 around.

[akinata]

for example if 4 armies are attacking an ennemy the game will auto select the 3 armies which will participate. and you cannot choose the 4th one instead. this is a massive problem, especially when an allied army (player or vassal) get caught up in the fight screwing you over. we should be able to see all available armies in the battle menu and select whatever we want!

 

[Crimsonraziel]

Are you sure all 4 armies were within "the zone" from the attackers point of view? I'm pretty sure I had 4 armies on the screen with the one that was deselected by default being the single scout. I didn't try to change that, because why? But I could swear I had the option to do so.

 

[MrButtermancer]

Had the same issue. All armies were clearly inside the battle zone.

 

[akinata]

Are you sure all 4 armies were within "the zone" from the attackers point of view? I'm pretty sure I had 4 armies on the screen with the one that was deselected by default being the single scout. I didn't try to change that, because why? But I could swear I had the option to do so.

the game auto select for you and you can't do anything about it except making sure you have only 3 armies around instead of 4+.
but then another problem arise when one of the selected armies is an ally/vassal one...

 

[gunnergoz]

I'm just picky about where I place my armies before a siege or fight, but that's just me...war is unpredictable for real, ya know...

 

[Dreepa]

Yes.
+1

 

[The Founder]

Note that Armies inside cities or outposts can not be selected as reinforcements, unless the target army is inside the same area.

That could be most tripups (and is a vexing design decision all it's own).