Did you accidentally tank your reliability or did something happen to give you a diplomatic penalty? Or did you get into a war with someone who asked all of his friends to join their war against you?
So there was nothing I could track that would explain it other than one thing, which was that Achilles was extremely angry. I have no idea whether his anger is what caused it There's next to nothing you can do about his moods other than not forcing him to end his turn garrisoned and not declaring war on your neighboring factions. There will be story questions that come up that you have to answer as Achilles' mood changes, but I was always very careful not to pick anything that had a penalty to diplomatic relations. And the factions that declared war weren't allies to one another or anything. It was just crazy. Actually, the reason Achilles started getting angry to begin with was that a neighboring faction declared war on him, and it just snowballed from there.
The other thing I had against me was "Great Power". I was far and away the strongest faction, which causes a lot of problems.
Seriously, you can survive an attack by so many factions? What does it take to lose?
I honestly don't know if I can survive it or not, but I think that I can. My advantage is that my armies are a hell of a lot better than theirs. I maximized my population growth and got to higher tier units before anyone else. I also targeted food and bronze settlements when picking my early enemies, so I have a decent number of armies, and I'd guess one of my armies is worth about 3 of theirs.
As far as what it takes to lose, in Total War Warhammer 2 and 3, you are racing other factions to a certain goal, but in Troy it's all about conquest, which is kind of a shame and maybe why they made the AI so aggressive. It's the only chance they have, really.