Delta Force 1's real terrorists join DOOM and DOOM II as a third faction: riflemen, snipers and grenadiers who fight you and the demons, drop Delta Force supply crates, and bring Delta Force 1's music.
What you get
Three terrorist types, straight from your copy of Delta Force 1 (its own soldier models, textures, weapons and voices):
Rifleman: jungle camo, M16 three-round bursts
Sniper: Russian, M40 rifle, long range, hits hard
Grenadier: desert, lobs M203 grenades that bounce
Opening firefights: every level starts with a squad already shooting it out with the demons near you. More squads wait deeper in the level and join in when you get close.
Three-way war: terrorists attack the nearest enemy, you or a demon, and never each other. Demons fight back.
Weapon drops: dead terrorists drop Delta Force 1 supply crates (ammo box, AK crate, grenade crate, sometimes a medic crate) that top up your DOOM ammo and health.
Delta Force 1 music in every level. Switch it off (and set squad size and extra squads) under Options → Delta Force: Hell Ops.
What you need
DOOM + DOOM II (Steam). Works with DOOM, DOOM II and Final DOOM.
Delta Force 1 (Steam). The first start reads its soldiers, sounds and music from your own install (a few seconds). Without it, the game tells you and plays as plain DOOM.