To elaborate a bit more: Diplomacy differs greatly from similar games (i.e. Axis and Allies, Risk) in that each army/fleet has the same 'strength'. There is no die-rolling involved to determine who wins in an encounter, it's just the number of units on offense/defense. It's impossible for a single army unit to capture a region so long as that region is occupied by an enemy unit, so taking territory requires either a concerted effort from multiple units that you own (which makes you weaker on other fronts), or it requires allying with other players in the game (which opens you up to being betrayed).
I wouldn't quite call it "the game that ruins friendships", but since it places a lot of emphasis on the social aspects of the game, rather than the strategic, it involves lots of lying and backstabbing, which can (understandably) strain relationships between the players.
I consider Diplomacy to be a brilliant game on par with chess or go - the only modern game I hold in such high regard. It's extraordinary that it's so well-balanced. Even Italy, the weakest power, has a decent win rate in online play ratings. The rules, with nothing left to chance but no alternating turns, make the game far more unpredictable than you'd expect.
And in my experience, good Diplomacy players do very little lying or backstabbing. The compulsive liar types can't get the strong alliances that lead to traction. A two or three way alliance that culminates in a later betrayal are the way to win.
It's a little weird, but being weak makes negotiating easier. You get more bidders, more or less. Say you're in a 3 way dispute, with two stronger powers. It's real easy position them against each other and you're the obvious ally. This works well enough that even the weaker starting positions seem to be more influenced by alliance choices than by their objective positions.
Austria is second only to Italy in difficulty. The central board position is a strength, but also a weakness. I'm a firm believer that Austria needs to crush either Turkey or Russia as quickly as possible, so they can get their back to a wall somewhere and not be constantly surrounded. Turkey is a better target, because it enables naval operations later. But ultimately it depends on your opponents.
Yeah, it's just hard to make Italy-Austria work, with the neighboring centers. Plus there's the ever-popular Lepanto. I love playing Italy, but if Austria offers me a Lepanto opening, I agree to it and then stab them immediately. Lepanto is a sucker's game.
I wouldn't quite call it "the game that ruins friendships", but since it places a lot of emphasis on the social aspects of the game, rather than the strategic, it involves lots of lying and backstabbing, which can (understandably) strain relationships between the players.