To infiltrate the base, you can kill the three enemies outside or avoid them: wait the sentinel to be walking to the left, than enter the gate, take the right (walking and, if needed, crouched), pass behind some boxes and head straight, to the left side of the base, where you'll find the entrance door; the tower watcher and the machine gunner won't notice your presence (if so, restart the level).

    Once inside, take the right to acquire the German officers uniform (you must kill an enemy to get it) then, to the opposite way, to acquire the level one papers; now, walk down the stairs.

    Show your papers to the first German that ask for it, at the end of the stairs; head to the right and, at the end of the corridor, again to the right; be quick, kill the German officer and pick up the level two papers.  If you wait sometime in this room, another enemy will show up but he seems to not be bothered by his comrade dead on the floor.  I'd say its artificial intelligence needs some improvement (fig. 1).

    Now, go ahead and show the level two papers to the second guard that'll ask for it; he's near a machine gunner.

    Open the door, go ahead, open another door.  Now, you must take care inside this next room because there is a third German that'll ask for the papers; he's located near some stairs.  The level two papers you have will not fool him.  If you don't want to kill him, just don't use the stairs, jump away from him (pic. 2).

    Keep walking through a long curved corridor and enter the next door, at your right.  Inside this room, you'll find the blueprints and you must kill another enemy to steal it.

    Exit this room, take you right, another corridor, two guys are chitchatting at the end of it, in German!  Now, I strongly advice you to kill both or they will be a problem, later.  Go upstairs, find a position where the two guys are in line, quick draw the hi-standard pistol and, with only one shot, you can kill both: it's cool!

    Enter the next room.  You'll find the StG44 and the stockpile.  It's time for explosion (pic. 3).

    Steal an StG44 and all the ammunition you can.

    Now, you have to set the explosive on its red flashing spot.  First, quicksave and reload your pistol.  Now, it's time for the trick: you have five seconds to get away from the explosive.  Use this five seconds to run to the nearest alarm switch.  Luckily, there is one very near you, just outside your room (pic. 3).  So, set the explosive and RUN in the direction of the alarm switch.  Stop when you're facing it (pic. 4).

    As soon as the stockpiles explode, since you're facing the alarm switch, keep trying to turn the alarm OFF until you make it.  It takes approximately three seconds, after the explosion, before the alarm can be turned off, so, keep clicking the use key.

    Again, as soon as the alarm is off, look at your left: three enemies are coming to get you.  Run to the nearest corner (pic. 5), crouch and kill them, using your pistol.

    Interesting is that, if you didn't kill every enemy on your way to the stockpile or even if you did, when exfiltrating, you won't find them inside anymore.  They're gone for good.  You'll be almost alone inside the base.  The exception is one machine gunner and one enemy inside a room that was previously with the door locked (pic. 6); the latter has health and ammo near, so kill him to get them.

    So, get outside, pass through an open gate (previously closed) to the left, pass through the watch towers and some enemies on the other side of the base, through the exit gate and bye bye.  No one is going to try to kill you: they still think you're one of them (pic. 7).

    Okay, if you're bored with the situation, try to kill them all.  You'll have a lot of fun.  I killed them all, even the tower watchers, but stealthily, whenever possible.  There were times, though, that it was impossible to avoid some respawning  Since you bought the game to kill some smart enemies, why not waste some time trying to kill them all.  A reader, codenamed Tretch Axebiter, sent me his mission statistics screenshot; he used the good old run & gun approach (pic. 8).