I remember back when I was learning to play Diplomacy in middle school, our first game, it was me and six of my better friends, learning in a free-hour from one of the teachers. I was Italy, my buddy Tom was Austria, and we said we'd work together.
Well as it happens, Tom was a bit gullible, as I knew from being friends with him for years, and I decided as Italy I needed to gut Austria quickly, so I stabbed him, hard. I had promised support into Greece, and I said I'd go after Munich. Hence I had armies in Tyrolia and Venice, and a fleet in Ionian.
Come the fall, his fleet goes into Greece, bouncing the Turkish army. His Budapest army, which had gone after Romania and been bounced, tried for Serbia, succeeding, and his Vienna army, which was foolishly in Bohemia (to get me Munich).
Well, by some stroke of luck, my stab worked great. His Bohemian army sat there, wasted, and I took Trieste and Vienna. My fleet grabbed Tunis.
In the end the game stopped with Austria and Germany dead, as many do. France was declared "the winner" with his 9 SCs, or maybe it was England, I forget, but my Italian party had been ruined by a Juggernaut, though I not yet knew its name.
Anyway, Diplomacy face-to-face is best when you know your fellow players, and it can be very interesting.