So, I didn't do this, but this happened to me once in Europa Universalis IV.
We were playing 4 people online, although I was stronger than the other 3, though could not entirely assert dominance. So a second player invited me into an alliance to take down the 2. strongest player, and seeing this as a great opportunity, I accepted his offer. His main asset was his powerful Navy which prevented me from Attacking him, so we ganged up our navies and prepared to attack him. I sailed my fleet from the Northern Sea into the Mediterranean. All my 150 ships that had taken me the past 50 years of game-time to build. And finally we see the perfect opportunity, the 2. strongest player just had his navy leave port, and we could intercept it before he could return to port. We declare war and prepare to fight this guys' navy of 200 ships.
But wait, something wasn't right. Why didn't my "ally" join me... Wait what is he doing? Did he just peace out with my opponents leaving my navy to fend for itself?
Wait what? Did he just join THEIR alliance and attack me O_o
Frankly speaking, this betrayal was huge, with all three of the other players sinking my whole navy, and proceeding to invade me. And then we had a war that lasted 30 years of game time. This lasted around 5 sessions of 5 hours of play. And it all started with the vilest act of villainy and betrayal that I have ever experienced.