I am completely at a loss here. I have been trying to siege Alexandria for the last 4 turns. I have Johnston in the region with a big army and he is activated and has attack orders (both buttons orange). He's been like that for the last 2 turns (the first turn he fought a battle which he won and the 2nd turn he was in defence mode as he was not activated. But why hasn't he started the siege the last 2 turns? The Union forces siege me all the time... I'm sure there is a completely sane explanation here - but I just can't see it... :-) I have an image but don't know how to attach one here yet.
I think I have found the answer myself :-) It looks like there is no siege option - as the union army is not inside the city. The lack of action is apparently due to a lack of engaging the enemy. I assume the union army has been skillfully avoiding battle the last few turns.
Yes, there was no sieging because the enemy had entrenched outside the city and my army had not succeeded in starting a battle yet. I put it on assault mode next turn and then the battle came. Alexandria is now mine but 5 turns later the union has built up several massive armies on the other side of the river and I'm streched out along the same line in handfuls of divisions - about half of their size the whole way. Winter has set in and causing losses to my troops. I have a bad feeling...
Siege is an automatic status, occurring when one side (defender) has >= 1 stack inside a region's structure (city, fort, etc.), while the attacker has >= 1 stack in the region, which is not in PP (Passive Posture), measured from the start of a turn.