A common theme among recent games here has been that no one can decide what Stratagems or buffs are allowed to affect. So let's look at the facts and evidence.
Counter- Offensive stratagem specifically mentions it applies to your own units.
"This Stratagem is used right after an enemy unit that charged has fought. Select one of your own eligible units and fight with it next."
Insane Bravery stratagem is by its own definition only applicable to your own units.
"You can automatically pass a single Morale test (this Stratagem must be used before taking the test).
Command Re-roll originally was ambiguous about its scope.
"You can re-roll any single dice."
Yet
GW clarified it's intent while being quite specific that it applies only to re-rolls.
"Q. Can I use the Command Re-roll Stratagem to re-roll a dice roll made by my opponent?
A. No. Note that the rules assume that a player always rolls their own dice (instead of asking their opponent, for example, to roll hit rolls, saving throws, etc. on their behalf).
That being the case, you can only use the above-mentioned Stratagem to affect dice rolls you make, and not those made by your opponent."
Other Stratagems lack such clarification. The only requirement for their use is that you qualify for them by army list. This does not necessarily stop you using them on your ally detachments.
"If your army is Battle-forged and includes any Chaos Space Marine Detachments (excluding Auxiliary Support Detachments), you have access to the Stratagems shown here, meaning you can spend Command Points to activate them. These help to reflect the unique strategies used by the Heretic Astartes on the battlefield."
Each Stratagem is limited in what it can affect by its own text. Such as:
"Endless Cacophony - Use this Stratagem at the end of your Shooting phase. Select a Heretic Astartes Slaanesh Infantry or Biker unit - that unit can immediately shoot again."
Yet if you're playing with allied detachments that include Heretic Astartes Slaanesh Infantry units, can this apply to them? Is your commander overseeing an entire force tactically?
What about allies in general? Can you apply such a Stratagem to your ally player's Heretic Astartes Slaanesh Infantry unit? Is your commander capable of lending his experience to friends?
How does this affect your view of buff auras? If my Lord applies rolls to all nearby friendly <Legion> units, do other detachments or allied players that meet the requirements count?
If I'm using Guilliman in a megabattle with all kinds of friendly
Imperium units from multiple players and detachments near him, do they all benefit from Master of Battle?
But what of your enemies? If they too meet the requirements of a stratagem, can they be affected? Sometimes it's not beneficial to buff your enemies. What if it is?
"Tide of Traitors - Use this Stratagem at the end of your Movement phase. If you do, pick a unit of Chaos Cultists and remove it from the battlefield. You can then set it up again wholly within 6" of the edge of the battlefield and more than 9" from any enemy models, at its full starting strength."
What of psychic powers? Some clearly denote enemy models are the target. Others leave it open to anything that seems to meet the requirement. Do these apply to allies and/or enemies?
"Diabolic Strength has a warp charge value of 6. If manifested, select a Heretic Astartes model within 12" of the psyker. Unit the start of your next Psychic phase, add 2 to that model's Strength characteristic and 1 to its Attacks characteristic."
Typically when an effect carries negative consequences, it's quite clear you must select one of YOUR units for those consequences.
"Chaos Boon - Use this Stratagem at the end of a Fight phase in which one of your Heretic Astartes Characters (not a Daemon Prince) slays an enemy Character, Vehicle, or Monster. Roll 2d6 and look up the result below."
But what if it doesn't state you must select one of YOUR units for those consequences? Can you "buff" your enemies and leave them wishing you didn't? Or estrange your allies with begrudging "gifts"?
"Enhanced Warriors - Fabius Bile can enhance one unit of Heretic Astartes Infantry (but not Characters - they refuse the dubious honour of Bile's gifts) that is within 1" of him at the end of any Movement phase. Roll a D6 for each model in the unit; the unit suffers 1 mortal wound for each roll of a 6 (only the strong survive Bile's expiremental cocktails). Then roll a D3 and refer to the table below to see what bonus the survivors gain for the rest of the battle. A unit can only be enhanced once per game."
-
So where does all of this leave us? What are the mechanics - buffs, strats, auras, powers, or abilities - actually able to affect?