This question is leaning a bit into alternative history, but I'll try to offer an answer, which is that it if Japan was not involved in China, then it likely would not have ended at war with the United States. While the Japanese and Americans had spent much of the early 20th century eyeing each other up across the Pacific, and there were undoubtedly some tensions due to American treatment of Japanese immigrants, the key event which provoked the irreconcilable difference between the two powers was Japan's ever expanding presence in China. I talk some in this post about why Japan went to war with the United States, and you'll note that nearly all of the factors tie directly or indirectly back to the war Japan was waging in China. Pictures of Chinese civilians fleeing Japanese bombs helped turn American opinion more hostile to Japan, while Japanese disregard for the Open Door in China drove American attempts to use economic warfare to reign Japan in. Indeed, the key moment which solidified the always fractured Japanese leadership around the Southern Operation and war with the United States and the European colonial powers, the Japanese occupation of southern Indochina, was itself a continued result of Japanese efforts to isolate Chiang Kai-shek's Nationalist government from outside aid. Fundamentally, the war in China was what drove the Japanese and Americans on a collision course to war. While, in the absence of the war in China, it's unlikely that relations between Tokyo and Washington would've been cordial, it is also unlikely that either side could muster the political will for war with the other. To go beyond this would be too speculative for this community, but I think this answer is sufficient to highlight the overriding importance of the war in China to the eventual outbreak of the wider Pacific War. To remove the war in China would be to remove the impetus for war with the United States.