There are two things that make Diplomacy so unique and challenging. The first is that, unlike in most board games, players don’t take turns moving. Everyone writes down their moves and puts them in a box. The moves are then read aloud, every piece on the board moving simultaneously.
The second is that prior to each move the players are given time to negotiate with each other, as a group or privately. The result is something like a cross between Risk, poker, and Survivor.
I played this game in college and loved it. The key is you can’t win the game without cooperating with at least one other player but you also can’t win without screwing over at least one other player. The “when do I screw over my friend?” is the most fraught part of the game.