War games eh?
Lt. Polpolion says "Holding ships in reserve is a wise tactic, even if my force is outnumbered at the outset."
Always designate some forces as reinforcements, but do not hesitate to use them too.
If your initial battle lines start to crumble, a concentrated deployment of the reinforcements can turn the tide of the battle. But if you delay the reinforcements too long, the enemy will have a chance to destroy your main deployments at leisure, gaining a foothold in both occupied area and morale, and you'll be unlikely to recover the engagement.
This is why a good reinforcement should consist of units with rapid response time and firepower. Heavy fighter wings or gunships are ideal for this role, although fast capital ships with good weaponry definitely make for good heavy metal flanking units, should you have a surplus of cap ships.
Meanwhile the main deployment units should have good staying power so that they aren't immediately swept away, which gives you as a fleet commander some grace time to make sound judgement as to when and where you should deploy your reinforcement units.
Where your units start to lose, deploy the reinforcements or part of them. Conversely, where your enemies' battle lines start to show signs of wavering, concentrate your efforts there. If you manage to destroy or rout units at one point of your enemy's deployment, it will disrupt their plans, reduce their ability to support each other, and also wreak havoc on their morale (which is sadly not simulated in FS2_Open...).
Also, as far as war games go, remember this: If you do not know what to do, flank them. If you do not know where to flank, flank from the left side. Almost uniformly, any firm decision will turn out better than indecisiveness.
At the wargames, Lt. Polpolion says, "I will commit my Bellerophon and Chimera corvettes only once enemy warships have been committed. Attacking installations allows me to force this ideal situation." Discuss.
Choosing the battlefield will give you an immediate advantage, as you are in control of the engagement and can select to either withdraw or continue the engagement at your will. Therefore offensive strategy is favourable to defensive. If your enemy has fixed locations they must defend, they do offer an excellent way to force your enemy to commit into an engagement under your rules, or forfeit their assets at that location.
Of course, this only truly works if your supply lines and forces are sufficient to defeat the defenses that can be mustered to protect the installations. If you can not mount large enough assault to provide decisive victory, the enemy installations can just as easily turn into death traps for your fleet as you continue to hammer them, yet they do not fall. In this case it becomes a war of attrition, something that is undesireable for both attacker and defender; war of attrition is usually devastating to unit morale and economy on both sides. It has sometimes been utilized by ruthless commanders who simply have the leisure of vastly outnumbering the enemy - however, even they usually find it very inefficient to simply use massed attacks against capable enemies using carefully formulated strategies and positions.
Conversely, even if your forces are outnumbered, you should still not settle on a defensive warfare, but actively harass the attacker by striking at their logistics, and if possible separating portions of the hostile forces from their main forces, then wearing the isolated units out by continuous attacks. (Again, sadly, fatigue is not modelled in FS2_Open...)
However, this is hard in FreeSpace warfare due to jump capabilities - the best you can hope is to lure the enemy into a system through a jump node, then blockade the node so that no supplies can be delivered to the isolated group, but this requires that you have at least two systems at your control and that your enemy can be hoodwinked into taking the bait.
If this is not possible, then your best choice when being outnumbered would be to use hit and run attacks against more or less isolated parts of the enemy fleet and evacuate as many of your fixed installations as possible, so that you do not need to necessarily stretch your resources to hold the installations. Loss of an installation can be a blow to industrial capacity and to morale, but getting the capable people out of them will leave you an option to abandon them, should it prove necessary, ie. if the enemy is trying to force you to meet them at a battle field of their choosing and the value of the installation is outweighed by the price associated in keeping it.
Of course, this could potentially enable you to manufacture traps of great cunning - by masquerading almost abandoned installations as fully functional, you could perhaps lure the enemy troops where you want them, while still retaining the option to disengage and abandon the installation if the battle does not turn into your favour.
...And from that day forward, any time a bunch of animals are together in one place, it's called a ZOO!
Unless it's a farm.