We often think of the great patriotic war in the soviet union as one of the most brutal and turning points of the war, as they were brutally slaughtered by the Germans, yet they turned the tide and fought back all the way to Berlin. However China was fighting against Japan for almost a decade (1937-1945). China was brutally destroyed in the first year and lost many of its coasts and the north was utterly lost. Japan was also far superior to china in terms of military technology and commanders. But Japan never fully conquered China. Why?
I am rather curious to understand your statement "China was brutally destroyed in the first year." Yes, the Chinese forces had sustained heavy losses, and many coastal cities and the capital (Nanjing) were in Japanese hands, but they had inflicted similarly heavy casualties on the Japanese, who were less able to afford them. In fact, despite capturing the secondary capital (Wuhan) in 1938, the Japanese suffered extremely heavy casualties and most of its combat formations no longer had the ability to fight with the initial tactical superiority they had earlier. This was shown in 1939, where the Japanese endured a string of defeats across the entire front as their troops, already suffering from poor logistics, had to deal with newly mobilized Chinese forces which were continuing to be supplied by American and Soviet aid, along with weaponry from the industries that had been relocated to the interior. In addition, the Japanese were also starting to run low on their stocks of strategic materials like scrap metal, oil, and rare earth minerals. Ultimately, the Japanese army-and the war machine in general-relied on winning short decisive wars, and the campaign in China was already being drawn out with no sign of conclusion. For the Japanese, an urgent matter was to close the backdoor route into China through the Burma Road, where supplies could flow to China. While the British did close the road under diplomatic pressure on several occasions, it nonetheless was a major concern for the Japanese.
In effect, the Japanese were unable to really force the issue in 1940. They did not have the resources nor the logistics capacity to successfully launch an offensive, while the Chinese were growing stronger day by day as they made good their losses and were taking in more supplies from foreign aid. They needed to shut down the supply routes into China to starve the Chinese military, giving time for the Japanese to build up strength and force concentrations for an offensive. Meanwhile, the Chinese knew that the Japanese grew less and able to wage war every day-hence, Chiang Kai-Shek demanded a return to pre-war borders before he was willing to negotiate. This of course was something the Japanese refused to do.