In our timeline Italy declined to help the Central Powers during WW1 ,and in 1915 they joined the Entente.But what if they do help Germany?Will almost nothing change or will this change the war entirely?
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.
It's worth noting that italy and the Ottomans didn't get on to well at this time.
Italy joining the central powers in 1914 may mean neutral or entente Ottomans.
While Italy as a central power would have a big impact on the war the change in the balance of power could be reduced if the Ottomans no longer join the central powers.
If Italy and the Ottomans both join the central powers I would go so far as to say that Italy joining in 1914 would have been war winning for the central powers.
Italy distracting France could mean Germany winning the race to the sea. If not navally Italy could be a major threat to the entente in the rather Mediterranean (assuming Austria is willing to operate at sea at least somewhat).
A little pressure is taken off Austria-Hungary which allows them to do a little more damage in the balklans and against Russia, nothing major just accelerates Russia's collapse by a week or two.
The big effect would be an Italian front in the south of France. 1914 was a pretty close thing, the diversion of even a few French divisions could result in Germany capturing Paris.