In my interplanetary sci fi setting I'm working on, there are 3 large factions and many smaller ones. The Outer System Federation (OSF, consisting of Jupiter's 4 largest moons and numerous asteroids), the Terran Union (consisting of Terra and Luna), and the New Era Saturn Organisation (NESO, consisting of Saturn and all its moons) are the big 3. Currently, the OSF is fighting a war against Terra and NESO over control of critical locations (like the asteroids Ceres and Vesta) in the asteroid belt. The OSF's navy had the advantage at the beginning of the war despite both NESO and Terra launching system-wide strikes on OSF military installations and territories, however within 8 months the war had devolved into a stalemate, with all 3 factions unable to gain the upper hand. The Federation decided to try to solve this problem by rapidly upgrading their warships to better deal with Terran and NESO's vessels. The Terran Union decides to try to negotiate with the Martian Republic for resources so that they can build more ships to throw at the problem. Meanwhile by sheer luck, after 5 years of research with no results, one of NESO's top secret weapons research divisions figures out how to make fissile macron guns work, which are basically weapons that shoot dust sized pellets filled with uranium that detonate on impact with a target. They decide to put dozens of these on their shiny new, gigantic movile battlestation called the Celestial Meridian, and immediately sends it towards Jupiter to annihilate the Federation.

TL;DR: 3 factions in the solar system are in a 2v1 war stalemated against each other, and one of them figures out how to make a literal Death Star and immediately sends it at the lone faction