In 1927, Jiang Jieshi a.k.a. Chiang Kai-shek (q.v.) tried very hard to eliminate the Chinese Communist Party, which we will call the CCP. This is hard to understand because in his expedition to unite China, he was an ally of the CCP in the United Front. The majority of his conservative supporters were frightened of communist influences in the Nationalist Party, and wanted to get rid of it.
It will not come as a surprise to learn that in the China of the Twenties many Nationalist generals and leaders of commerce were in fact anti-communist. So of course were Japan and the Western nations. They feared communist attempts to seize Western-owned settlements.
The CCP was very well established in Shanghai, that great and beautiful city/port, controlling labour unions, influencing the all-important students and so on. Paramilitary units had been formed to monitor strikes (and foment them), and kill opponents foolish enough to disagree over policy. It had all but seized Shanghai just as the Nationalist Army was approaching. But in April, 1927, very shortly after being welcomed by the workers, Jiang turned on the communists with all guns blazing. The Japanese Consul-General supplied the cash, aided by Chinese businessmen, and Jiang simply used the flourishing Shanghai underworld to round up and murder communists, their supporters and families.
The later to be very famous Zhou Enlai was among those captured but he escaped with his life. Oddly enough Jiang was not actually in the city when these events took place, but in Nanjing where had gone to purge more communists. For those who would like to learn more about this coup, it is recommended to read André Malraux’ book La Condicion Humaine.