Play online with a friend here!
Diplomacy Chess was inspired (as you may have guessed) by the strategy game Diplomacy. The main focus of Diplomacy is on making and breaking alliances, but what I found the most interesting when I played it was the movement/combat system where players all secretly submit moves for the turn, and then those moves are revealed and resolved all together. It is very difficult to effectively counter everything your opponents might do, so making your moves forces you to decide when to play conservatively, when to take a calculated risk, and when to try and predict exactly what your opponents are thinking.
I was curious how simultaneous turns might affect other strategy games, so I created a chess variant to test it out with some friends. Originally, my variant followed the movement rules of diplomacy much more closely, allowing pieces to move away from attempted captures. That variant was fun, but very punishing, since one mistake could allow the queen to go on a rampage unchecked.
I eventually changed the rules to the ones explained on the game page linked above, which I think this much better captures the spirit of Diplomacy combat. Since a successful combat in Diplomacy usually forces your opponent to retreat rather than removing their unit, it’s fine to allow a player to choose to retreat outright, but because removing pieces is a normal part of chess gameplay, it makes sense for that to be the default in Diplomacy Chess.