Second Sino-Japanese War
After six weeks, the Japanese Army left Nanjing.  It finally occupied the industrial cities in southern China. Chiang moved his capital city from Nanjing to Chongqing, and began a tactic of "using space to trade for time."  He was able to spread Japanese soldiers out too thin.  This means they were covering too much space with too few soldiers.  This made them harder to fight, and made it easier for partisans to fight back in occupied China.