A thought experiment. No need to get into how this scenario would develop.
In a one vs two fight, starting on August 1st, 1914, with Germany on one side and France and Russia on the other, which side would prevail in the end?
Neither side can receive alliances or assistance from any other nations on the planet. However, involving a neutral country in the war by having one of the combatants invade them for one reason or another (a la Schlieffen Plan) is permitted.
Who would win?