1 Siobhan: Hey, is that someone else joining the party? That’ll make seven of us. We can play Diplomacy!
2 Martian 1: What is Diplomacy?
3 Siobhan: It’s where you make deals that you pretend you’ll keep, and then you backstab everyone until you’re left ruling the world!
4 Martian 1: Oh, sorry. I thought you were talking about another board game.

Diplomacy is a venerable board game first published in 1959, which has since become a classic. It was inducted into the Academy of Adventure Gaming Arts and Design Hall of Fame in 1994.

Diplomacy is a fairly simple wargame, in which seven players control various national factions in a vague simulation of World War I Europe. The board is a map of Europe, extending to Russia, Turkey, Syria, and a small part of northern Africa. The map is divided into regions of unequal size - for example, Norway, Sweden, and Finland are a single region each, whereas England and France are both divided into six regions each.

The rules are very simple. Each player begins with three military units (or four for Russia) and must move them to capture various capitals, either ones that are neutral at the beginning of the game, or that belong to other players. Each map region can contain only one unit at a time. Every two rounds of play, each force counts the number of capitals it controls, and adds or removes units to match that number. Players are eliminated by having all their capitals captured, and the lone surviving player wins.

The crucial element is that all moves are simultaneous, and are written down by the players without knowledge of what the other players moves will be. All moves are revealed and then resolved according to a few rules that dictate what happens when two units try to move into the same region.

Alongside this is a second crucial element: players are given a significant amount of time between rounds—while planning their moves—to discuss tactics, plans, and alliances with the other players. These discussions can include announcements to all players, or pairs or larger groups of players can retire to discuss in secret without involving other players. This is where the game really happens.

Diplomacy isn't really about moving pieces on a map. It's about making alliances, about negotiating mutual assistance pacts with other players. No player can win on their own - you need allies. And they need allies too, so there is a strong incentive to make alliances. But you can't ally with everyone. At least not for long. Typically, you play friendly with some players, and you pretend to be friendly to other players, while actually planning to betray them, sooner or later. The trick is to be friendly with enough players to build yourself into a position of strength before they realise it, and betray them before they inevitably betray you.

As such, it's a highly social game, that rewards skilful negotiation, bluffing, and playing enemies against one another. A typical tactic is to negotiate an alliance with player A, and gain a guarantee of what their next move will be by offering them (false) details of your own next planned move, and then immediately going to player B and telling them what player A is planning and how player B can take advantage of it, with your help. If done right, you can get players A and B to attack each other, while you sit on the sidelines and keep pretending to be friends to both of them, while planning to pick them off one by one after they've weakened each other.

It takes a cool head to prevail in a game of Diplomacy, as well as a willingness to betray everybody, and to be betrayed by everybody. Among groups of players with the right mindset—that it's just a game to be enjoyed for the fun of the negotiations—it can be the most amazing experience. However, among groups of people who don't take well to being blatantly and gloriously backstabbed it can result in sore feelings and the breakdown of friendships. This has given it a rather notorious reputation.

But it is an absolute classic, and rightfully so.