Why were the Ultramarines fighting Salamanders?

Question

I'm re-reading Rules of Engagement, one of the Horus Heresy short stories and I'm confused.

This is the story of the 4th Company adopting the Codex Astartes and following their Primarch's tactics.

What I don't understand is why they're fighting (and killing) the Salamanders in one of the engagements? Both legions remained loyal!

I did consider that this may be a training exercise

like the last one where "Horus" turned out to be Roboute Guilliman in disguise

But I don't believe that's the case as they've previously fought Death Guard and World Eaters who are most definitely traitors!

Why were the Salamanders the target during the engagement? Were they a traitorous company or was something else going on?

Answer 1

They were all Ultramarines in disguise, it was a huge training exercise

The whole point behind Rules of Engagement was Guilliman wanted to test out the Codex in different scenarios, against traitor marines and also against loyalists.

If you read carefully, you'll notice no one actually dies. I think the only way you know someone is incapacitated is when their helmet light goes out? I can't remember that specific detail, but I know there is some proxy for death that doesn't involve any marine being dismembered or blown up.

Towards the end, there are Ultramarines missing from a huge battle on Ultramar, I think the reasoning behind that was to let us know the traitor marines were actually Ultramarines in disguise.

So both the traitor marines and the loyalists are actually Ultramarines in disguise.