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The French attempted to bring Japan into alliance with themselves as early as 1883, although as the military situation for France improved they dropped the idea. During the war, Japan used China's distraction to ferment a coup in Korea that ultimately was put down by the Chinese.
France was also willing to help the Japanese undo the unequal treaties with the British.
What if Japan had formed an alliance with the French in 1883 and fought against China with them?
I would imagine the Gapsin Coup would be a success (meaning a pro-Japanese modernizing Korea) and France might have the additional heft to grab the Pescadores and Taiwan (which they wanted to do but failed at historically). Japan meanwhile likely gains some other territory than Taiwan - Lushunkou/Dalian perhaps?