The novel I keep thinking about with this is 'A Game of Thrones'. Martin constructs alliances not as simple plot conveniences but as constantly shifting fault lines. The Stark-Lannister dynamic is the obvious starting point, but it's the quieter, more desperate pacts that really twist the knife. Take Catelyn freeing Jaime to get her daughters back. She's making a deal with the literal murderer of her husband, driven by a maternal hope so strong it blinds her to the political fallout. That single act fractures the Northern coalition, undermines Robb's authority, and sets off a chain of distrust that contributes directly to the Red Wedding. The alliance wasn't just dangerous; it was existentially corrosive, eating away at the very loyalty holding the Stark cause together.
What's so brutal is how the characters are often fully aware of the poison they're swallowing. Tyrion's alliance with the hill tribes to defend King's Landing is a masterpiece of this. He knows they're unpredictable and their loyalty is bought with hollow promises of future spoils. He uses them because he has literally no other option, and their savagery wins the battle but permanently stains his reputation and his soul. The 'danger' isn't just external betrayal; it's the internal compromise, the person you have to become to hold a pact built on sand. By the end, you're not just fighting your enemies; you're managing the monster you invited home.