What do you do when placing the Mission first requires you to leave a fallen comrade?

If you are receiving hostile fire, whether in an offensive posture(moving to contact) or a defensive posture(NDP or firebase) stopping to care for a wounded Soldier takes you out of the fight and endangers other Soldiers in your unit. Mission first in this case means call for a medic and carry on. If you are disengaging or being extracted from a hot situation then bringing back your fallen comrades is part and parcel of your mission.

Without proper context, you do both at the same time - PART of your mission is to care for the "fallen" - which means to protect them from further injury, do triage, etc - the situation will dictate which you do first and obviously, you can't help them if you're dead.

A fallen comrade IMHO is not someone that has been hit but KIA. At the point it is mission first. Much like on D Day the troops were told not to stop and help wounded on the beaches but let the follow up medics care for them as the mission was to secure the LZs. I read the ethos I will not leave a body without ever effort to recover the remains. But, that is my take.