[Squirrelloid]

Movement happens before combat. All movement. The only time combat seems to happen first is when two armies move into each other, at which point it has to decide which province they fight in, should they encounter each other.

So, when you break siege, if the sieging army moves away you will never catch them. If you move an army onto the fort from the province they are moving to, and it decides the combat will be fought in the fort's province, then your units breaking siege will get to join in.

One other thing, there can be two battles in teh province in one condition. If you attempt to break the siege (either with armies moved in from elsewhere or with units in the castle or both), and they attempt to storm the castle in the same turn, then if the sieging army wins the regular battle any remaining commanders set to storm will attempt to take teh castle. This will be reported as a second battle and will be fought against any units in the castle who did not attempt to break the siege. (Ok, you could also get a magical phase battle in addition to both of those, obviously)