Selling intelligence seems to be one possibility. Even temporary allies dismembering the same victim may be interested in the disposition of each others' forces and tactics, since sooner or later, they're going to fight each other. And if one can use anonymous attacks that 'suggest' other players (particularly from paths your nation is not considered strong in), it may be possible to disrupt things.
There's also scorched-earth -- making your lands not worth conquering, by going nuts with pillaging, uber taxation, and bane venom charms, but that's not to be done lightly.
Heh. Playing naive doesn't always work. Back in graduate school when playing odd games like 'Cults Across America' and 2-player 'Settlers of Catan', one of my most frequent opponents would reflexively treat me as the major threat, reject all offers at trade and treaties, and attempt to wipe me out ASAP. Might have had something to do with my predilection for rapidly commencing warfare (attempting to wipe out all enemy leaders on the very first turn... once almost managed a _Cults_ victory in setup phase, but after that he started paying more attention) and for nasty combos (ex -- in Catan, you have 2 VP and opponent already has 3 VP due to one extra province including ore and brick... buying additional province, playing Sir Lackland to get knight token, and playing Border Dispute to swap Sheep for opponent's extra Ore region. Heh. Best done with a Scout card so you can get Brick/Sheep, so you have a Sheep region to trade.
)
It got to the point where in the 2-player card version of Catan, it was customary to be loaded with nasty action cards early on and to be very cautious about being the first player to the 3rd victory point, because the opponent might then get HIS 3rd victory point (cheapest = Sir Lackland, heh) and then open fire.