Imperial Japan was as successful as it was in China largely because of how divided China was at the time, although Japan's modernization played a major role as well. China in the early 20th Century wasn't a unified political body as we think of most states today. The Revolution of 1911-12 in China was successful at overthrowing the Qing Dynasty, but the end result was more of a failed state as China devolved into competing zones of control of different warlords. Competing governments were established based out of Beijing in the north and Guangzhou in the south, with what would become the Nationalists (KMT) gradually fighting its way north to become the officially recognized government of China by 1928. However, the KMT had a fairly tenuous grasp on authority, and civil war would continue off and on pretty much until Japan invaded China proper.

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For reference, Japan had a presence in China since the end of the 19th Century, and after WW1 they had control of the Kwantung Peninsula (now Liaodong), the former German possession of Qingdao, and a major presence in Shanghai. In 1928, the Kwantung Army assassinated the largely independent warlord of Manchuria and three years later executed a false-flag attack to manufacture casus belli for a full invasion of the region. Although Japan received much international condemnation for the invasion and subsequent installation of a puppet regime in Manchuria, they retained control. From then on, it was a matter of the Kwantung Army provoking and escalating border incidents with the KMT until the Marco Polo Bridge Incident in 1937 cascaded into a full-blown war.

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In military terms, the Chinese were massively outclassed. As part of the Meiji Restoration, Japan had recognized the dire need to modernize (in large part thanks to China's misfortune in the Opium Wars), and in the subsequent decades they had assembled a modern army and navy. The IJA had standardized around a western-styled army by the end of the 19th Century, and the IJN had assembled a formidable force of capital ships. Equipment was largely standardized, and despite interservice rivalries both the IJA and IJN had arguably the most capable land, sea, and air forces at their disposal in East Asia by the 1930s. Compare that to China, which was embroiled in civil war, lacking any real standardization of equipment across its myriad of semi-independent entities with no navy, no domestic aircraft manufacturing, and barely any air force to speak of.

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The KMT made efforts to improve the situation once it was clear war with Japan was in the future - they openly received aid from Nazi Germany and the Soviets. The Germans were notable for supplying arms and helping set up factories, as well as training some soldiers in modern German fashion, while the Soviets were more notable for supplying aircraft. But neither provided enough support to make up for the massive disparity in organization, training, and equipment. When war broke out in 1937, China threw the bulk of its German-trained elites into the battle at Shanghai, where they were largely annihilated by superior numbers and Japanese airpower. Though the KMT had aircraft from America and Russia, they lacked the training and numbers to seriously compete with Japanese air forces of the time, and very early on in the war Japan had attained air supremacy over much of China.