Pick elements before or after battlefield is set up?

Hmmm...an interesting idea Derek.
However, in your example while still on the coast of Northern Gaul and not yet loaded
into the invasion barges, how would the Romans know which troops they would need?
In other words, how does a general know in advance which troops to bring with him?

Now it is true that at the Battle of Mons Graupius in 84 AD the Roman general Agricola
kept his legionaries out of the engagement and placed them in reserve (perhaps because
the Caledonians were on a Difficult Hill where his Blades would have been at a disadvantage
against Warbands), so he sent his Imperial Auxilia against them instead.
But he still had to march to reach that battlefield, and might have met the enemy in the open,
so he brought his legionaries along ‘just in case’.

On the other hand, DBMM army II/12 Macedonians allows both Alex the Great and his dad
Philip II to deploy some of their Phalangites with javelins (so 4Ax) instead of being armed
with their usual sarissa (so Pikemen)...useful when facing say Thracians and Illyrians lurking
in harsh terrain. Unfortunately the DBA army lists doesn't allow this.
But it does seem a bit odd for a Macedonian commander to decide in advance whether
to leave their pikes in camp and take up javelins before the general has even seen the
type of battlefield he will be fighting over!

I think it would be better if those troops that can dismount before a battle, those that can
dismount during a battle, and some special troops that were capable of being re-armed
(such as the Phalangites mentioned above), were allowed to be deployed as their alternative
types once the terrain has been placed but before either side has begun to deploy.
That way the general can see the battlefield, and then plan on how they want to fight it.

Of course, not every element is capable of being re-armed or dismounted in this way...
...just limit it those few troops who historically did so in reality.

One issue is that giving generals options AFTER terrain is picked seriously disadvantages those armies with minimal element options.
Generals have to work with what they have, not cherry pick to suit the whims of the moment.
(For the last 30 yrs I’ve been playing these rules that was not how it was done...it works fine ‘as is’, TBH.)

Interesting.

Thanks for all that number crunching Stevie! Interesting to see the liklihood of predicting the terrain based on the agression of each army.

And thanks too, Martin. "One issue is that giving generals options AFTER terrain is picked seriously disadvantages those armies with minimal element options.
Generals have to work with what they have, not cherry pick to suit the whims of the moment."
- See... I always thought a good general did everything to stack victory in their favour before the battle even started. As John Steinbeck famously said, "If you find yourself in a fair fight, your tactics suck!"

But I suppose this brings to mind the difference between strategy and tactics. The general might not have control over the strategy, but it's his role to do the best tactically with the situation he is put into. The greatest miliatry leaders of history had a clear vision and were in control of both strategy AND tactics. I'd guess the majority of generals were given orders from above and had to make the best of their situation, good or bad.

As for 'convention', I recall reading somewhere that the British really upset the Europeans in the 15th Century because we didn't follow the same military conventions and chivalric etiquette as they did. They really didn't like it because in their minds we weren't 'fighting fair', but we thoroughly beat them. If I get time, I'll try and specifics, but it was years ago when I read that, and I can't remember where...

"(For the last 30 yrs I’ve been playing these rules that was not how it was done...it works fine ‘as is’, TBH.)"
- Perfectly willing to follow everyone else's experience here, which I why I posed the question in the first place. It comes down to the purpose for the game. Obviously most games and tournaments are designed to be fair contests that both sides can enjoy. But as Jim said, the order of army composition/terrain isn't specified in the rules, so I still think it is an interesting tweak to 'convention', even if only for friendlies and solo matches.

(Oh, and in the example which prompted this discussion, the Romans were destroyed by the Britons anyway. The warband were across the field in no time, scurrying across hill and through wood, and the Romans legionaries were caught in a column desperately trying to reach the only open bit of field near the middle. The Auxilia protecting their flanks were certainly good to have-and more useful than more legionaries would have been-but the dice gods hated the Romans on that day. It was all over pretty quick.)

You have to pick army composition before working out attacker/defender and terrain, because some armies have different aggression values depending on composition.
For example IV/13b Medieval German Army is Aggression: 1 if Militia Spears, else 2.