Play consists of alternating phases of negotiation (when players go into furtive huddles, form alliances and plot ) and action (when players simultaneously write their instructions on a piece of paper). The instructions are then revealed, and moves executed -- typically to cries of "You promised me you would attack Berlin!" or "I don't believe it, my best friend has just taken Paris from me!"

The game ends when a player has captured 18 of the board's 34 strategic "supply centres" or, since the average game lasts eight hours, when everyone is too tired or bored to continue. [...]

After graduating in 1953, Calhamer had been intending to do a PhD, with the eventual aim of joining the Foreign Service; but Diplomacy distracted him from his studies and he dropped out to develop and market the game, which he later sold to a games manufacturer. By the time he applied to join the Foreign Service he was told he was too old.

Instead, Calhamer found work as a park ranger at the Statue of Liberty and later became a postman in La Grange Park, Illinois, where he supplemented his income with royalties from his game.

An amiably eccentric man who enjoyed working out the prime factors of car number plates he passed on his rounds, Calhamer would sing his children to sleep with The Battle Hymn of the Republic and posted a notice above his cat's bowl which read: "It is essential in this life that you be your own cat."

Although he was an honoured guest at Diplomacy tournaments, he was far too nice to be proficient at his own game.

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