One of the aspects of the Tienanmen Square crackdowns I've never really understood is the role of the 27th Army Group seemingly massacring police, other Army groups, running over army medics and police lines in APCs and shooting soldiers and executing wounded soldiers.
From what I've read, they basically had to be cornered on a bridge by other army groups and put down.
What I've never seen explained is why, why did this Army Group spend so much time attacking police and other military, Government emergency services personal? Why was it left up to other Army Groups to stop them? Were they rogue?
Hi there! I took a course on the Tiananmen Square Massacre a few years back, and based what I could find in the articles and books I still have laying around I think I can answer a small part of your question, and hopefully help build towards a more comprehensive one.
To start off with, I have studied the Tiananmen Square Massacre from a social history perspective, investigating primarily the student movement and its successor in the form of the June 4th Movement. This significantly colours the understanding I have of the event as a whole. I'm particularly poorly versed in the higher-echelon politics of the PRC, so bear with me.
I think the most important starting point to gain an understanding of the actions of the 27th Army is on 20 May 1989. In the night of 19 on 20 May - almost 2 weeks before the massacre took place - the PLA attemped to quell the demonstrations after martial law had been declared (govt. internally) late at night on the 19th of May. Troops mobilized from many different provinces, totalling close to 200 000 men converged on Beijing to quell the demonstrations. A large contingent of troops came from the 38th Army, whose commander - Xu Qinxian - was opposed to the intervention and refused to carry out his orders. This army was among the best-equipped and best-trained in the PLA, which caused concerns among the military leadership of insubordination or even mutiny. The three key figures that would have given Xu Qinxian his orders were Deng Xiaoping, Yang Shangkun, and Zhao Zhiyang. These three headed the Central Military Commission, which had all had to approve the order to intervene and start the crackdown. Citing the lack of Zhao Ziyang's approval for the order, Xu rejected advances to persuade him otherwise by Deng and Yang, asked for sick leave, and was court martialled. A politically reliable commander was appointed, and the 38th Army was on its way. Zhao Ziyangs 'opposition' in this case was caused by the fact that he had been politically sidelined after attempting to negotiate with the students on Tiananmen Square the 17th of May, prior to the declaration of Martial Law. On the 20th, as martial law was declared, he was removed from the Central Military Commission.
Many of the other troops outside of the 38th Army were reservists, very often coming from poor or even illiterate backgrounds. At dozens of transport/traffic arteries leading (in)to the city they were halted by huge crowds of demonstrators and symphatising civilians. In some cases this resulted in the complete surrounding of units, severing entire units from their logistical- and supply train. Many of these troops were 'turned' either temporarily or permanently by the crowds, as water and food was distributed, and the surrounded troops were effectively conquered through a mix of civil disobedience and logistical warfare.
Another - slightly more odd - issue the 38th Army had, was that it had also served as training unit for reservists from the Beijing boroughs, leading to many chance encounters of ex-comrades-in-arms and a shared sense of summer-soldiery cheer in what was not a bad May-month.
The 20th of May also saw the signing of a letter by 7 retired generals of the PLA, which further reinforced the paranoia about mutinies and insubordination within the Central Military Commission. As the 21st of May passed with many units surrounded or blocked from entering their assigned positions, the first truly violent clashes started to occur on the Fengtai District, where soldiers were pelted with stones and in some cases isolated and beaten. On the 22nd of May the unit closest the square of the 113th Division of the 38th Army attempted to negotiate with the student leaders to allow them passage and to empty out the square. By 24 May the PLA leadership ordered the retreat of all units from the city, and to isolate in barracks until further notice. In practice this meant that scores units were considered completely unreliable and prone to mutiny due to their 3-4 day confrontation with had started becoming more and more like an anti-regime revolution. Effectively, all the units that had been in the city during those days, were confined to re-education regimes to build political reliability. The declaration of martial law was by this point still not widely supported in the higher echelons of the PLA, where especially the Navy produced a large number of dissenting officers and endorsements of Zhao Ziyangs perspective. Some naval cadets ended up joining the students, and in other parts of the Army and Air Force soldier-councils started forming, petitioning their commanders to not use force against the demonstrators.
Deng Xiaoping summoned the military leadership to his residence on 2 June, and declared that he had enough, and that the operation to quell the demonstrations and the movement must proceed immediately, without reserve. The 27th, 65th, 24th, and 63rd Armies were dispatched to the city and ordered to infiltrate it starting in the evening of 2 June. This occurred through a variety of avenues, but primarily unmarked buses, plainclothes movements on foot, and through public transport arteries underground. The 27th mustered about 7 000 men in the Great Hall of the People and Zhongnanhai, and the 65th some 10 000. About 6 000 men of the 63rd Army were available at the square on the evening of June 3rd, and the 24th Army took up residence in the National History Museum and the Ministry of Public Security. One of the issues of the infiltration was that many soldiers did not bring their equipment to avoid arousing suspicion, which was ferried in in some cases by trucks and helicopters.
The 38th Army also returned to the fold, being held up at Fengtai. The 27th Army meanwhile was commanded by the nephew of Yang Shangkai, who was a politically reliable and seemingly loyal military leader. As the 38th Army was held up, it ironically caused the first casualties of the night of 3 on 4 June. As the evening progressed into the night, the unarmed units of the 27th Army started positioning themselves outside of the Great Hall to intimidate the crowds. The 38th Army meanwhile moved through the boroughs, killing many civilians whilst executing its orders to advance on the square. Other units from supporting elements saw mass desertions, the burning of own vehicles, and the abandonment of equipment, and the first clashes between PLA units were reported in the early hours of June 4.
As the square was cleared and possibly as many a 10 000 individuals had been killed on the square as well as in Beijing as a city, the military insubordination had not stopped. In fact, it had escalated, as the post-Massacre reprisals and violence escalated before that. Insubordinate units clashed with loyal units, and in some cases civilians and students were once again involved. Much of the blue-on-blue violence was committed by the 27th Army, given their political reliability. Unsuprisingly the 38th Army and the elements from outside Beijing that had 'disloyal and antirevolutionary' components suffered significantly at the hands of the 27th Army.
It was not until 7 June (!) that most of the internecine violence stopped.
So long story short, it basically boils down to political paranoia about military insubordination; mixed into the large cocktail of social revolution, westernisation, and simmering class warfare tthis causes unseen scenes of internecine violence within the PLA.
My sources for this are primarily:
EDIT: Excuse my poor anglicisation of Chinese personal- and placenames, it's been a long time since I've engaged with this topic and the language..
EDIT II: I am so very happy for the engagement with this topic! Honestly quite surprised it gained so much traction, but given our contemporary circumstances it is very important we engage with this truly horrible event in an objective manner. In no small part we have been able to do so here because of the intervention of the mods, for which I am thankful. I'll try to answer as many follow-up questions as I can today, but alas, it is also a workday which means that some of your questions will remain answered until weekend has started.