You get over it. I played a game here with my wife; she couldn't *wait* for the chance to screw me

It seems obvious, but remind people that it's a game.

If you're playing Settlers of Catan with a friend and you steal one of their resource cards, would they then conclude that you are a thief and likely to steal other things from them outside the game?

Of course not. Because everyone understands it's a game. Diplomacy is no different. It's just a more emotional game so people get more invested in it. A reminder now and then never hurts.

Reconsider your choice of friends if they can't separate games from reality.

Ever play poker with friends? Diplomacy has lower stakes, but the dishonesty usually bites harder.

Set expectations up front.
But also bear in mind that if they don't have a good time then they probably won't keep playing, so it may actually make sense to go a bit easy on them in their first game.

Yeah. I usually try to be a little nice to people. I'll stab them, but maybe a year or two later than I would have normally, so that they get a chance to get going, stab me first, etc. I've had some friends very effectively take advantage of the latter, incidentally.

Interestingly, I had a friend who wouldn't play with his fiance, because he didn't want to be in a situation where they would have to lie to each other. This made some amount of sense to me.

I introduced a friend to WebDip and *he* killed *me* out of his first game (a World game) and went on to a 3-way draw.

I've introduced 5 friends to this game now, and have enjoyed mercilessly raining supremacy down upon them all. I tell them its an extremely cutthroat game upfront. No mercy. They should have believed me.

I've introduced maybe two dozen people to webdiplomacy. I've stabbed most of them at least once. ;)

Just preface the explanation of the game with "this is the game where you betray your friends and have fun doing it and there is no bad blood because everyone knows thats how the game is played and if you cant handle that dont play. Ok? Good. So you have armies and fleets, and they can hold, move, support hold..."

I was introduced to the site along with a bunch of others and we all played a few games together. Everyone (including the guy who introduced us, but I think that's because he started college and got a girlfriend) except for me quit within a month or so due to the tension it caused in everyday interaction.

abgemacht: Yep, I think that's the way to do it. ;-)

Of course, a little warning prior to the game wouldn't hurt: 'You are agreeing to play a game of Diplomacy. Please be aware that, if I get my way, you will watch all your hopes and dreams die before you. You have been warned.'

Yellowjacket: "They should have believed me." ha ha ha ha!

Together with all said above (mostly, being easy no them a turn or so so they get the hang of it, and really explaining up front it's only a game but they are going to get betrayed and they are going to betray), I'd add you should give them a keyword (like "rules question") which they can use to ask you questions on game mechanics and such, and to which you will reply without considering your own benefits and without benefiting from it. That'll give them some confidence in the game (not you) and make it easier for them to enter and enjoy it.

Don't play together so you can work together in each other's games... then learn his tactics and use it against him in a game with him ;)

There's one other thing that hasn't been mentioned:

" I " am not playing diplomacy. It is the fictitious character "redhouse1938" that plays diplomacy. And he's not playing himself. He represents the nation that was randomly chosen for him.

That means there are two layers between my "RL person" and "me in the game."

and how about in RL?
if i'm England i don't feel like i represent the nation, i feel like i have those 3 SCs at the start as home SCs...

It's much more fun to wait until they can put up a decent fight before crushing them mercilessly.
@Sepherim - good point about implementing a way to separate understanding of the mechanics from the diplomacy.