When Colonel Max Bauer doesn't die from fever in a Chinese hospital in 1929, there's a good likelyhood that something like a Sino-German alliance might occur.
He continues reforming the Chinese military, lets his good connection to the German arms industry play, and finally on a visit to Germany convinces Hitler that China is the ideal proving ground for tactics and armament.
German technology and know-how are transferred, Krupp and Rheinmetall set up production facilities, Chinese officers are trained in Germany and German instructors train the Chinese troops.
Result: A far better performance of the Chinese, frustrating the Japanese even more than they did in OTL. So, the Japanese might start looking for other targets quite earlier than they did in OTL.
But part of the reason the Allies aren't going to be hit by Japan is that the Allies are going to be forced to support Japan over China in one shape or another, assuming that this situation is anything more than a simple German-Chinese friendship.
In the case of just a continued German/Chinese Friendship, the allies are going to be in a pickle. Can they attempt to support another Chinese Warlord against Chiang? Can they come out of the diplomatic situation without having to fight either China or Japan? Can the Allies persuade Chiang to their efforts instead of Germany? Certainly the Logistics make it pretty much impossible for Germany to remain China's supplier after 1939, although perhaps the Soviets can strike a bargain with both Parties.
The Critical Question I foresee is whether the USA and UK are forced into supporting Japan, even if that support is nothing more than turning a blind eye to Japan's actions. If China is clearly attacking Allied assets, there is no question of the outcome. If China is doing something else, like basing German Aircraft for bombing the Dutch East Indies or otherwise showing a co-beligerent attitude, I would think that the Allies would have to swallow dealing with Japan.
The Japanese CAN NOT attack the Allies if they are supplying Japan with War Materials--and that's why the war is likely to be very different. Japan needs those war materials...and it needs to cut Allied Support for China. If China is an Axis power, or a co-belligerent state leaning Axis, the Allies will both keep the trading ties to Japan open and cut support for China. In both Cases, Japan will not escalate the war to include the Allies.
Its the third situation--a Pro German but Neutral China--that I'm having a problem deciphering. If Finland is the model to use of a friendly state on the wrong side of the war, I can imagine that the Allies would probably either attempt to mediate a peace deal or since that is likely to fail, slowly kill trade with Japan while attempting to bring Chiang into their orbit. That would be close to OTLs situation, except that Japan might be very reluctant to augur the full wrath of the United States alone--Japan was prepared to fight the Soviets in exchange for the German Declaration of War against the USA, and I would interpret this as a recognition that Japan would really rather not fight this war--and its possible that this is enough of a butterfly that the war doesn't happen because the USA gets into a war against Germany in Early 1942 and the USA relaxes its pressure on Japan as a result.
It's a race situaton--will Japan choose war before the USA is already embroiled in one and is willing to negotiate?