The Doomguy in Doom 3 and in the Resurrection of Evil Expansion Pack were both named "Marine."Evidence for this lies in the fact that nobody, throughout either game, calls them by any else — except in the ending sequence for Doom 3, in which he is referred to as "Corporal" by someone who has no way of knowing his name! Furthermore, the marine in Resurrection of Evil is referred to more directly as "Marine" by Dr. McNiel. This implies that they are related; perhaps the Resurrection of Evil Doomguy is an older brother or cousin.
The protagonists had help.Seriously — how easy would it have been for the forces of Hell to simply keep sending an infinite number of enemies until the character was overwhelmed? Or turn the two protagonists into zombies, as they had with every other soldier on the base? In fact, the hellish "visions" received throughout Doom 3 were just such attempts to zombify the main character — but they failed due to some sort of intervention, either divine, or by some opposing faction at work in Hell, or some other force which was opposed to Hell's invasion of Mars but couldn't or wouldn't work openly.
Alma is on Mars!In one of the Alpha Lab levels, if you enter a side corridor you hear a creepy female voice whisper "They took my baby." followed by a scream and some paranormal activity. The voice is eerily similar to Alma's, who says pretty much the same thing to you at the end of F.E.A.R..
Doom is a playable segment of ancient history within Warhammer 40,000Experiments in teleportation involving trips to hell, check. Demonic possession and corruption, check. Chainsaws, check. BFG's, check. Badass Normal taking on the daemons with guns and winning, check. The list goes on...
The Doom 3 Marine died in the initial attack and went to an Ironic HellThe hell being endless corridors filled with demons and monsters and no hope, his only human contact the PD As of people he had already blasted. And the occasional hiding human.
The Marine is a direct descendant of the Martians.In the latter levels of the game, there are several theories that suggest the Martians who survived the demonic invasion and didn't sacrifice themselves to create the Soul Cube fled to Earth. The Marine could very easily be a descendant of those refugees, for a couple reasons shown in the course of the game:
The Martians are the Ancients
Betruger is an Ori
The Doom 3 Marine is B.J. Blazkowicz revived from cryogenic stasis, and Betruger is the reincarnation of "Deathshead" Strasse.Depending on which continuity you go with—with the series either ending with Wolfenstein 2009 or with the alternate history of The New Order—B.J. Blazkowicz volunteered to use the technology recovered from Castle Wolfenstein to have himself frozen in cryogenic stasis, to be awakened if the technology should ever be used to threaten mankind again. Centuries later he was found in the castle ruins by Union Aerospace forces, but the degradation of his capsule caused problems with his revival, which cost him his memories. The executives of the UAC—having known of Blazkowicz and his history—immediately saw the potential use for him as a one-man army, and he was trained in the elite military under the name John Kane. One single act of defiance led to his transfer to Mars City, something Betruger didn't expect... Dr. Malcolm Betruger not only bears a bit of resemblance to Wilhelm "Deathshead" Strasse—bald old scientist with two different eyes—he also shares Strasse's brilliant mind and keen affinity for research and development. Betruger is also a German-sounding name. When Betruger first entered the portal to Hell found in the ruins on Mars, he came face to face with the vengeful spirit of "Deathshead", who the Icon of Sin had given command of its demonic armies for its invasion of the mortal realm. "Deathshead" possessed Betruger and was fully reborn in his body, sent to open the gates of Hell and unleash the invasion. The one thing "Deathshead" hadn't counted on was his old enemy—the amnesiac B.J. Blazkowicz now known as John Kane—being transferred to Mars. If you're going with the Doomguy's name used in the Doom RPG, then somewhere along the line B.J. possibly got his memories back and met a woman who he started a family with. This would eventually lead to Stan Blazkowicz, the Doomguy, who would continue his predecessor's battle against the forces of Hell.