At the end of Know No Fear, Erebus explains that the Word Bearers real main objective was to take the Ultramarines out of the Heresy. Their intention was to cripple the Ultramarine war effort and cut them off from the rest of the Imperium.
At the end the Word Bearers very arguably accomplish both. 4/5ths of the Ultramarine fleet is mentioned to be destroyed and at least half of the Ultramarines at Calth are killed, with possible more dead. A large chunk of the Ultramarines are left standard on Calth while the Warp storms close in. Meanwhile the Word Bearers and the World Eaters prepared to attack the rest of Ultramar.
From a strategic point of view I could certainly argue the Word Bearers won, and depending on how you interpret the book, they chose with withdraw, rather than the Ultramarines forcing them out.
I wish to hear people's thoughts on this.