Here is a rather underhanded offensive strategy. Leave one of your outposts along your border with Person A undefended(although if you have a smuggler to rush defences there that's even better). Make sure all other ones are defended. Launch an attack at against person A's bases, making sure you'll take at least one. Hopefully, A will see your undefended base, and attack it. The easier it looks to take, the more likely an attack. Tell A that you changed your mind about attacking him, and that if he gifts you the subs he sent, you'll gift the ones you sent, or you'll return his when they arrive. Say whatever you must to convince A that he should gift you his subs. If you succeed in convincing A to gift, send a sub towards him that has his return "gift". Tell him you scheduled it to gift right before it arrives, so that he can't change his mind about peace, or so that a pirate can't steal it. Then just never actually gift it. All the subs you said you would gift will attack A, and hopefully you'll take some outposts.
This works best if your subs will arrive after his do. It's also good if you have a way to reinforce your undefended outpost, because your deceit may not go as planned. This strategy requires a lot of trickery and outright lying, so it's not something you want to do a lot, but it has its uses. Obviously, this won't make you friends. It's something that kind of has to happen within the first 2-3 days, and it works best if you can eliminate person A as you do this.
"You want to believe that there’s one relationship in life that’s beyond betrayal. A relationship that’s beyond that kind of hurt. And there isn’t."
-Caleb Carr