It is harder to justify the better at attacking then defending as I can't really see much that could not defend but attack very well, everything could defend of not just attacking as a means of defending themselves if need be.
I could perhaps see this from some alien fiction perspective, but I still feel it to be a bit strange set of skills.
Berserkers
WWII Imperial Japan 'Banzai!' charge
WWI Arab Revolt cavalry raids
Finnish light cavalry 'Hakkaa Paalle' tactics
Greek city states' Peltasts
Roman auxilliary slingers, skirmishers, Numidian cavalry. . .
Fire ships
Fire camels!
German and/or Soviet troops hopped up dexedrine / methamphetamines / who-knows-what
Retiarius gladiators (versus Myrmidons, anyways)
Grenade-carriers (Not Grenadiers, with their centuries of tradition and esprit de corps and high morale; but illiterate peasants given a couple/few greandes and told which way the enemy is.)
B-C-N shells / bombs / missiles (which you probably don't want to deploy over your own territory)
Suicide bombers
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Basically, I think the training to inflict shock & disarray and ignore casualties while charging forwards is very different from the training to sit & endure and wait patiently for the most effective moment. I think both have their place in an army, but not every stormtrooper needs to be a sniper, and vice versa. It's certainly going to be faster to train someone for only one style of combat.
But that is tactical application of a force as aside from the more operational side that Aurora portray.
Attack is a form of defence from an operational stand point. Anyone who employ a completely static defence and no mobile reserves will eventually be destroyed no mater what. Just because something can cause chock does not mean it is not perfectly usable during defensive operations. This is why elusive enemies are so powerful in defence and why a mobile defence is often much stronger than a static defence in general.
If you look at the number of men you need to hold of an offensive force it is not going to decrease just because you have a line full of "berserkers", they just fight tactically different from a pikeman.
That is why I find it peculiar that a unit can have a higher offensive operation value than what they can have while defending in general as attack is a form of most often effective defence.
The best defence in any firefight on the modern battlefield is not armour, it is suppression firepower. If the opponent can't fire at you, you don't need armour in the first place.
The difference in strength from an operational standpoint is the mobility of a unit. The more mobility a unit has the more effective it is in a dual role to support both attack and defence. This is why static units such as Garrisons are so bad at offensive operations. Sure they could potentially be used for some offensive operations, but they are so ill equipped for it that no one would ever use them as such. That does not mean that they will not use attack as a measure of defending themselves in a tactical perspective.
I think I just have a different view of tactical versus operational units strength and weaknesses.
I would generally say that any units that are so one dimensional they they attack no matter the consequences are really weak units as they are extremely predictable by the enemy from an operational or strategical perspective and will be destroyed accordingly by superior firepower. Numidian cavalry (as an example) was not really an offensive formation, being elusive is a very strong defensible characteristics from an operational standpoint as you can harass enemy supply lines and movement from a strategic point of view and thereby restrict enemy operational movement.
In my opinion having a line of heavy infantry standing their ground on a hill, thereby claiming the hill is not operational defensible characteristics but a tactical one. Skirmishers that delay or prevent an enemy army from moving out of an area is a great example of an effective defensive operational use of a formation. The same is the guerrilla fighting formations of VC forces in Vietnam where the elusive nature of the forces meant that winning a battle one day meant loosing the same battlefield tomorrow without the opposing forces even having to fight for it.