Short answers, pulled from the story.
The Second United Front was an alliance formed between 1936 and 1947 to unite against Japanese invasion. It involved a ceasefire between the ruling Kuomintang and the Chinese Communist Party to focus energies on fighting Japan.
Zhang Xueliang and Yang Hucheng conspired to kidnap Chiang Kai-shek on the 12th of December 1936. This event forced a truce that allowed both parties to suspend fighting and form the Second United Front.
In July 1937, the Presidium of the Central Military Commission ordered the Red Army to reorganize into the National Revolutionary Army. The force was structured into the New Fourth Army and the Eighth Route Army under National Revolutionary Army command.
The alliance began breaking down as the CCP intensified efforts to expand military strength through absorbing guerrilla forces behind enemy lines. By December 1940, Chiang Kai-shek demanded that the CCP's New Fourth Army evacuate Anhui and Jiangsu Provinces, leading to further conflict.
Communist forces were ambushed and defeated by Nationalist troops in Southern Anhui on the northern shore of the Yangtze River in January 1941. This clash became known as the New Fourth Army Incident and effectively ended any substantive cooperation between the two parties.