For those who haven't played but want an understanding of Diplomacy, here's how it was described to me:
It's like Risk without luck, but instead of taking an hour it'll take seven, and instead of being mad at your opponent at the end for a few days you'll hate them for the rest of your life
The way it's played is that there are seven players who are effectively equal in power. There is a timed period where you can discuss your intent freely with others(as open or closed as you'd like).
During this time you strategize, plot against, make deals and offer support.
At the end of the timed period you write down your moves, they're put into an envelope and opened all at once.
No single nation is more powerful than another, you almost always need another's support to make any progress. That said, you don't have to stick to your word, and neither does your opponent.
Did you promise Germany you'd help them into Warsaw, but also promise Russia you wouldn't? It doesn't matter both were plotting against you the whole time because they overheard you talking with England about how you were going to stab both of them in the back.
It's a really fantastic game that has unlimited re-playability.
There's also an urban legend that it was JFK and Kissinger's favorite game[0][1]
[0]: https://boardgamegeek.com/thread/1529003/kissingers-favorite...
[1]: https://www.reddit.com/r/boardgames/comments/6je8nu/til_henr...