Had a crazy night last night, and maybe one of the slickest maneuvers I've done in any game.
Me and a friend start in a sloop reapering. We come across another sloop fairly quickly, take down it's mast and I board it killing the occupants while they yell something about an alliance. I don't pay much heed to what they're saying, but stay on the ship to make sure it sinks. Turns out this ship was going to ally with a Brig, and it happens to pull up right next to the Brig who still thinks it's a friend. I fire into the Brig from the commandeered ship and it starts to take some big damage, and I hear a lot of confusion on their side. They think the people they were going to ally with betrayed them, perfect! I board the ship with the intent to kill, but they don't notice me and they have NOTHING so I decide to let them live. So I just start playing an instrument as they sink their former ally's ship.
At this point they think I'm the ally, so I just hanging around but they're being cautious of me. Eventually I tell them what we did, and we're just some randoms who broke up their alliance. They were so confused, but we tell them they have no loot and we'll be back when they do.
We go off reapering, and later we notice a fort is lit up. They were doing the lantern quest, and we head there with the intent to kill them. We happen to get boarded by one of them, he causes a bit of fire, apologizes when he sees us then asks if we want to join the alliance to do the fort. We ally, clear the fort, get a ton of treasure. There's a slight mishap when I hear "Hey be careful wit...*BOOM* *BOOM*" they accidentally blow up their ship with two nukes. Our ship takes a bit of damage but survives, so now we have to load 5 people an all the treasure on a single sloop. We eventually make it to Reapers and cash in 200k.
A good night and a great story.
tldr: Broke up an alliance by a taking over a ship without the other ship knowing and attacking them with that ship. Later befriended them and made lots of money.