With the OTL alliance system Germany is screwed, so OP is hard to have happen.
Once you change the alliance system, everything changes, so the war does not start with a Central Powers Attack.
More likely, it's going to be Russian aggressiveness in the Balkans or Caucasus or French/British colonial aggression.
If it is a British move, then Germany probably seeks a diplomatic solution.
If it is French or Russian, then it is probably a war Britain sits out of.
I think the balance of the war depends upon how
motorized the German army is ATL 1920.
German economic growth leading up to the war was exponential:
So, the question is, who stands to gain by a more motorized WW1? I think the army with the doctrine that benefits from it most. I have to say Germany.
Schwerpunkt , coined by the press "blitzkrieg," was nothing new. It was standard doctrine for years. After all, Germany beat France in 1870 pretty quickly. German doctrine was always to win a war in a summer, hitting so hard and so fast that they get a favorable peace. If the war is 1920, Germany is not changing to a long war doctrine. Culturally impossible.
The question is, where could they get the favorable peace? I think with a quicker mobilizing Russia, an even more built up France, and a scale too insurmountable with British involvement, Germany is essentially going to go Napoleon in the east and defense in the west--probably an ATL Barbarossa (which originally was a two-pronged offensive straight for Leningrad/Petrograd and Moscow--no Ukraine. The idea would be to crush Russian forces at the border and to race to the cities faster than Russia can form defensive lines. Whether this works really depends upon German preparations to convert Russian rails (which, ITTL is more favorable than 1941 OTL as there is more cautious preparation.)
As Russia ITTL will also have tanks and armored cars (they might have less than Germany ITTL) they will lack the doctrine. I also think, without years of trench warfare, tanks would not be massive behemoths, but more mobile and lightly armed. I am unsure if Germany had enough horse transport to bring things like fuel and ammo behind ATL armored columns. Ultimately, I think the war is decided if Germany can score a sufficient victory before reaching Petrograd. If they actually reach the city limits, Nicholas II sues for a British brokered peace and the war ends. This is not going to be Germany going to the Urals, so it has a chance of success.