Can confirm that a game of Diplomacy will take a group of happy friends and turn them into scheming backstabbers who will never trust one another ever again for the rest of their lives.
The solution is to only play Diplomacy with a group where everyone is a known scheming backstabber who will never trust one another ever again for the rest of their lives! Have tried, can confirm it works. Good times.
If you want to play in one sitting, in-person you need 6 or 7 players for a good 6+ hours...So basically it's a game for college students (I played it a lot at uni).
As an adult you can try to online versions where you only do one move per x(usually 24) hours. The games last weeks but you can play remotely at your leisure.
https://www.lifehacker.com.au/2017/11/diplomacy-the-most-evi...