Was Hillary Clinton smart to “press the reset button” with Russia? Can Donald Trump’s business-negotiation experience carry over to international relations? Questions about diplomacy loom large in politics, as they have through history. The closest game players can come to the experience is through Diplomacy, a classic simulation of international politics and conflict set in Europe at the start of the 20th century. Though the game is almost 60 years old, it has proved to be useful for researchers today studying the psychology of betrayal.

The game works best with six or seven players, and these days, you can find opponents on several Internet game sites. What makes Diplomacy unique is the role of collusion among the players. In games like poker, it is a grave form of cheating. In Diplomacy, it is required.

The players take on the roles of the great European powers of 1900. The rules encourage players to form alliances and make trust and betrayal vital social undercurrents of the play. Each turn starts with a 15-minute negotiation period during which anyone may say anything to anyone else, in public or—usually—in private. Next, players write down “orders” for their military units in secret. Then the orders are revealed simultaneously, the forces are moved, any resulting conflicts are resolved, and the turn ends. To win, a player must control 18 of the 34 key territories on the continent. Face-to-face games last about six hours.

Each country starts out with just three units (typically two armies and one fleet). Because of this, players have to support one another’s actions to win battles and expand their empires, which also earns them more units. Promises of military support feature prominently in the player-to-player discussions that give Diplomacy its name—but those promises needn’t be delivered. Since orders are private, players can never be sure that they won’t be betrayed.

When Diplomacy is played online, all negotiations happen in text, leaving a record of everything everyone said. A team of computer scientists led by Vlad Niculae of Cornell University took advantage of this data in a study published last year in Proceedings of the Association of Computational Linguistics. By algorithmically sifting through 145,000 messages from 249 games, they discovered patterns of communication that occurred right before one player betrayed another—that is, before a history of two players supporting each other’s moves abruptly ended with one attacking the other’s territory. Players who were about to betray an ally talked less about plans for future actions in the game, wrote more sentences in their messages and were more positive than they “normally” were.