1) Serbia is gone: they wouldn't be able to hold
against the austrians.
2) Russia and Austria would fight a long, bloody war in Poland. Difficult to predict who would win, as both countries were very weak, with poor economies (when compared to France or Germany), obsolete armies and bad commanders. The russians have more manpower but the austrians are a bit more developed and the war would happen nearer to the austrian supply centers. Probably they would suffer from mutual destruction.
4) As the war raged on, some military lessons would be learned: horse cavalry is useless against machine guns. Artillery and machine guns nests can stop mass infantry. The french, british and germans would learn, from this war, that they need very strong fortifications and replacements for the horses, like some kind of armored automobile with machine guns or light cannons. But in the sea, the lessons of the battle of Jutland wouldn't be learned so the great powers would still insist in light armored fast battlecruisers (like the ones that got blown in Jutland in our timeline) instead of going to super heavy battleships like the Missouri or the Bismark. Also, the danger of submarines wouldn't be realized.
5) Both the russians and the austrians would exhaust themselves in this war and their empires would crumble. Russia was alredy under revolutionary agitation since 1905, the austrians nearly fragmented in 1845. That war would be the last straw and revolutions would happen. The russian revolution would probably follow the same path it did in our timeline, ending with a communist government. The austrian would be nationalist rebellions shattering the empire. The balkanization of Austria would be important to the future.
6) France, Germany and Italy would struggle against each other, dividing the austrian and russian (Finland, Poland, the Baltic states) fragments and that struggle, a new Great Game, would pave the way to the next war. I predict that the germans go after the core of the austrian empire (Austria and Bohemia), the poles would ally with the french, the finns with anyone that would protect them from the Soviet Union (maybe the british?) the italians would go after what in our timeline would be Yugoslavia, annexing Croatia and Albania. Some territories would be annexed, others turned into puppets.
7) The Soviet Union would be a source of instability, exporting communism and threatening the old order. But noone would do anything about that because no one wants to risk a two front war. These communist movements would be a threat to the german empire, with it's massive well educated proletarian population but without a crisis to provide an opening to a revolution there would be no german communist revolution.
8) The US would keep growing in economic power, threatening the old european powers, and, together with the soviets, would sabotage the colonial empires. Both the soviets and the americans would be interested in dismantling the colonial empires, the US to open more markets, the soviets to acquire allies.
9) Since Great Britain wouldn't be weakened by a large scale war their rule over India would not weaken. They would make no concessions to the indian nationalists and the indians would rebel like the americans did (they even had their own version of the Continental Congress in our timeline).
Flashpoints for the true Great War in this new Timeline:
- The ruins of the austrian empire.
- German Empire deciding they had enough of the Soviet Union.
- The indian war of independence.
- Japanese conquest of China.
- American-Japanese rivalry.
Evolution of the balance of force:
- The french would soon reach parity with the germans and both would be locked in an arms race.
- The Unites States would become the largest economy in the world and would have to compete with the british for the control of the seas.
- The british would be drained by the war in India.
- The Ottoman Empire would stabilize, reform itself into a somewhat modern State and when the oil extraction in Arabia and Iraq begins they will start getting very rich.
- The japanese will keep conquering China until the US decide they had enough.
Possible alliance for the World War, assuming war for India:
GB will be hostile to anyone helping the indians and afraid of the growing US and german might. To counter Germany, they must ally with France and probably Italy.
The US needs to open a second front should war with GB comes. That means alliance with Germany and maybe the Ottoman Empire. The US needs someone to help them against the japanese. That means the Soviet Union.
PS: forgot about nukes - They would be developed. The discovery of atomic decay made that inevitable. The longer it takes for the Great War to happen the greater the chance that the americans, germans, french and british will have atomic bombs and will use them, at least against military formations. Since Europe is small and the cities are packed tightly even nukes as blitzkrieg bombardment will burn some cities. The japanese, on the other hand will bomb the chinese cities with nukes because they didn't care about the consequences of atrocities (in our TL they used chemical and biological warfare against defenseless chinese).