The line ups were a bit unusual, to say the least. On one side, you have some thousands of Communist party workers (probably not well armed) loosely aligned with several hundred thousand workers. On the other side, you have elite Nationalist army units, some 5,000 of the best trained and equipped elite troops in all of China, under General Bai Chong Xi plus gangs such as the so-called "Green Gang" led by Du Yue Sheng.
The four groups initially worked together to defeat the warlords holding Shanghai. The Chiang's troops and the Green Gang together turned on the Communists and workers on April 12, 1927.
Why did Chiang feel the need to enlist the Green Gang in these efforts? Couldn't his Army troops do the job by themselves? If not, what did the Green Gang add to tip the balance of power in Chiang's favor?
Or did Chiang Kai Shek feel the need to work with the Green Gang because he feared that his Army troops would not be able to counter a "three way" coalition of Communists, workers, and the Green Gang itself?
This question was inspired by a footnote to another question:
A reference to these events can be found in the link in the footnote of other question.