The Dual Monarchy loses a front, and the French gain a front. That's enough for Austria-Hungary to hold their own front without nearly so much German help, freeing German troops to hammer a France that has been forced to redirect troops to stand off Italy.
Depending how early the Italians join, this leads to anywhere between a decisive Central Powers victory (if Italy is sufficiently bribed to join in quickly, or if Germany fares better early and the Italians decide the Central Powers are going to inevitably win, leading them to join) to a minor Central Powers victory that's mostly a clear loss for Russia and Serbia.