Simply look up the vp-files.
It was the Hydra, part of Serkr indeed.
Which introduces the possibility that Serkr was split up for this operation, (which seems odd as they are supposed to work as a team) or they took some significant losses. Not wiped out clearly, but if the Hydra was there one has to ask why the Pilum and the Marcus Glaive were not
At the end of WiH, the war is effectively lost, with both 2FM and 3JRF being reduced to a shadow of their former selves. Supporting Byrne and whatever his endgame plan is is pretty much the only option they have.
Makes me wonder what black-ops trickery the Laporte & the Fedayeen (sp) are going to get up to in WiH part 2...
The strong inner Tev within me says 'hopefully nothing' xD
Also @ Mars (since i dont want to quote his massive casualty chart in order to save space haha) I think we can safely add the Indus as a possible loss (unless there has been info saying the indus makes it out) Because that ship is friggen torched, and in a decaying solar orbit when we leave it. I'm inclined to think that the Fedayeen would save the crew and not bother to attempt to tow the indus. They didnt bring anything capable of towing it with them, unless some freighters jump in right after the screen cuts away, and the Indus was on a pretty tight clock for the point of no return.