Second United Front participant Chinese Communist Party
The Second United Front (traditional Chinese: 第二次國共合作; simplified Chinese: 第二次国共合作; pinyin: dì èr cì guógòng hézuò; lit. 'Second Nationalist-Communist Cooperation') was the alliance between the ruling Kuomintang (KMT) and the Chinese Communist Party (CCP) to resist the Japanese invasion of  China during the Second Sino-Japanese War, which suspended the Chinese Civil War from 1937 to 1945.Background
In 1927 the Chinese Communists retaliated against Kuomintang following a betrayal of its members in Shanghai by National Revolutionary Army commander Chiang Kai-shek, which marked the end of the KMT's four-year alliance with the Soviet Union and its cooperation with the CCP during the Northern Expedition (aka First United Front) to defeat warlords and unify China.In 1931 the Japanese launched its invasion and subsequent occupation of Manchuria.  Chiang Kai-shek, who de facto led the central government of China, decided that China must avoid all-out war with Japan due to domestic turmoil and inadequate preparation.  Therefore, he "pursued a strategy of appeasing Japan while struggling for real national unity and over time sufficient strength to confront the Imperial army. This appeasement policy lasted for another six years". Even though his campaigns against the Communists resulted in their retreat and a 90% reduction in their fighting strength, he was unable to eliminate their forces entirely, and his policy of "internal pacification before external resistance" ((in Chinese):攘外必先安内) was very unpopular with the Chinese populace, which caused widespread resentment and demonstration against the ruling KMT leadership and its regional warlord allies.