China was neutral in WWI until 1917 when they declared war on Germany. Almost immediately after, they were facing difficulties with Japan who was also laying claims on the German-controlled parts of Northern East China (Shandong, specifically). After the end of the war the Allies awarded Shandong to Japan who controlled it until 1922. The foreign population of the foreign settlements in places like Shanghai also volunteered in great numbers to go fight for their countries, though there were mixed feelings about this back home in China as British citizens and German citizens were often in business together, with most businesses having co-ownerships, and most social organisations having positions set aside based on a sort of national quotas.

A quick read on the foreigners in China during WWI is Getting Stuck in for Shanghai by Robert Bickers, published by Penguin though possibly only in China I'm afraid. At least the version I have is marked as such. Might be able to find it online though.

During WWII, the war was already very much on their doorstep by the time the US got involved. A good overview is Shanghai 1937: Stalingrad on the Yangtze by Peter Harmsen, which admittedly is fairly limited geographically, but it touches on the battles in Tongzhou that occurred earlier on in the war (usually called the Second Sino-Japanese War round these parts). I won't speak too much on that since there are people here way more qualified to talk about WWII era Asia than I am.

edit: Someone just posted this video on /r/Shanghai. Colour footage of Shanghai in 1938 filmed by a Japanese businessman. Gives a good view of the impact the fighting had on the region.