Depends how big the threat is, how much indirect fire support both sides have, what the local terrain is like and the relative firepower of the two forces. Also you need to consider whether a second attack can be delivered by unseen forces from the North.
Easiest and 'safest' is to just use in-situ. This leaves only the extreme wing of the force defending the flank, and there is a risk of being outflanked if the enemy force cannot be stopped cold by direct and indirect fire, however there is no movement to give away the positions of each element, and all remain dug-in and reasonably safe from fires from all directions - there is no advantage to defining an explicit 'front' for in-situ though.
If the terrain is reasonably covered, either by being in dead ground or by virtue of local cover and concealment, then you might want to select the flank element and defend using this one in-situ. Part of or all of the rest of the force can be gathered into a new group and assigned to all-round defence, or successive lines and oriented on the threat at the appropriate position (behind or to the flank of the initially contacted force) - it is worth keeping a motorised element as an exploitation/counter-attacking force.
Perhaps using line formation for defence is not the best option - all round defence by strongpoints, with a mobile force in reserve may be a better option, even though it has slightly less firepower initially.
Once a unit moves it loses the benefit of the dug-in positions, but it will improvise the new position using some of the extant works if the unit is on a previously 'improved' position - this is at 10x the rate that a unit normally digs in - this makes it a safe manoeuvre if you have sufficient firepower to hold the enemy beyond assault range, and the enemy has minimal indirect fire support assets. Against an attack by armour or heavy artillery, the best option may be to just stay put in unspecified or in-situ mode.
In-situ is one of the poorer combinations of security firepower and vulnerability though, so it shouldn't be over-used for main defence positions (can be useful for after the first positions have cracked to prevent newly rallied (and recently re-dug-in) units attempting to move back to the original defence location in the face of overwhelming firepower).
It should be noted that the 'fire' command also sets in-situ by default, so I always set the duration to the minimum and immediately apply my desired formation and behaviours once it completes - it can be useful to simulate performing an action-drill on contact though.
For support by fire (and indeed for 'most' of my attacks against 'only' screening elements) I tend to use move with maximum aggression by battalion - to terminate the advance once in contact I can set a fire command, but this is seldom needed with maximum aggression, unless the weapon has very long range, needs to be deployed before firing and needs to be moved into LOS to commence engagement - here explicitly setting a fire command prevents unwanted displacement. Movement with a lower aggression setting permits more frequent advances, and can bring units into contact sooner, but they tend to retire if they come under 'effective' fire, even if no casualties are caused - it is important if you intend to come to grips to push the aggression setting up as the range decreases.
(Not explicitly related to HTTR, but this is how I played RDOA, but it seems to still work well in COTA).
< Message edited by Lieste -- 2/27/2010 9:24:49 PM >