(Ok, i know that i'm actually don't should reply to this, but the temptation is too big for me)
If the Ottoman Empire also entered the War Russia is in dire trouble with a three front war. Even if the Chinese performance is very mediocre, Russia is only just doing better than the Ottomans and the Austro-Hungarians and is slowly losing to the Germans. Needing to keep 300-350,000 troops actively engaged in the Far East would overstretch Russia's already pressed resources. And if China promises the French Indochina independence and alliance and sends arms and an army "of liberation"? France has to deploy significant additional resources to hold Indochina and either wouldn't have been able to commit troops to the Salonika front or the Near East. And Britain wouldn't have been able to raise the Chinese Labour Corps and wouldn't have been able to deploy as much of its manpower to the Western Front as OTL. And France would have had the same problem with Indonesia.
And that's only on the basis of China's Army being barely competent--Yuan Shikai or Marshal Zhang level. If they have managed to train up some reasonably good generals to the level of the best Taipeng leaders or Chiang Kai-shek's very best (never mind the Red Army in 1949 or 1950) and with German technical advice they will do serious damage on those fronts. And Britain won't be able to hold Weihawei or the Portuguese Macao.
If Japan entered the war on the Entente side or stayed a friendly neutral Britain might just manage to hang on to Hong-Kong but at a resource cost elsewhere. Those Anzacs wouldn't be available to send to Gallipoli - needed for the Cantonese and Annamese fronts.
I can see the following moves to China in this war:
- Immediate offensive towards Vladivostok and Khabarovsk (With some good generals in command this can be a
great hit at the russians)
- Siege of Hong Kong, recalling that even if we consider that economically, China is not in its full potential, it still manages to produce MANY SHIPS to have a navy capable of acting considerably better than other CP's navies due to the circumstances (A.k.a don't have the necessity to outplay the Royal Navy home fleet, just outplay the Australian and Russian navies. Improve the scenario a bit and you can see a chinese navy bigger than the KM)
- Indochina War 0.5, unfortunately, the australians will feel in blood why you should never give China a chance to support the vietnamese.
- Baikal and Central Asian campaigns, the russians will have to sacrifice the victory at some front to have a chance of beat the chinese in the others. The 3-front war is just 'cause the chinese is just one opponent, in reality is a 5-front war. With the chinese having luck to have an great officer pool to the war, the russians would be just beheaded.
- Naval War in the East, this would be great, Spee will have a LOT of friendly ports to play in the Pacific. And if the chinese naval tradition plays in and the Qing have competent naval officers, i can see the british regretting waking the dragon because of some boxes of opium
- This is almost impossible but....WAR ON THE HIMALAYAS? Jeez, this war is turning really global