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...

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.

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...