I have promised an explaination of the tragic events that happened in Tiananmen Square on 1989-06-10 for a while now and here it is.
Run-up to Tiananmen
Prior to 1989 Deng Xiaoping was making many gradual reforms, among them economic and political. The previously draconian measure imposed during the "Gang of Four" coup attempt were recinded, and "whistle blowing" was encouraged.
But the economic success brought with it large scale corruption amongst senior government officials and senior PLA officers. Why PLA? In 1981, when Deng announced the reforms, he slashed the military budget (in order to fund agriculture and social programmes) but encouraged the PLA to start businesses in order to finance itself. Unfortunately, some PLA officers would exploit their command of naval bases and use them to trade and undercut many state-owned and emerging private enterprises. This did not go unnoticed.
Foreign influences
Idealistic students decided to start a movement in early 1989 to inform people, and eventually mobilise them against these corrupt officials, much like Mao mobilised the people against Lin Biao (former defence minister) during the "Cultural Revolution." These "idealists" found inspiration in American-style mass protests of the Vietnam era, brought to them by Taiwanese and US intelligence agents, who happened to be "young students."
Why the interest by the CIA? The Berlin Wall came crashing down, the "Velvet Revolution" were coming about in Czechoslovakia and Poland, Nikolai Ceucescu was about to be executed and the USSR was near collapse. Quite simply, the CIA couldn't resist a chance to overthrow the other major pole of Communism.
So the "students" in China started to camp out in the early spring on Tiananmen Square. Initially they enjoyed great support amongst the populace, where parents and children alike would come to applaud these marches. Initially the government decided to let the issue die down or negotiate a solution, but after 2+ months of camping out and literally polluting Tiananmen Square (both with bodily excrements and garbage) the situation urgently needed to be dealt with by government officials.
Key players & their views
Zhao Ziyang, the Prime Minister and hence no.2 official in the government, was initially sympathetic to their cause. He had spoken with some of the "leaders" and said he would promptly address the matter in cabinet, with a probable investigation. But he also stated that as Prime Minister, he had a duty to ensure public security and thus had to bring in police and army troops into the area. These troops were mostly local conscripts from Beijing and were thus sympathetic to the locals and PM Zhao.
Li Peng was the minister in charge of public security and no.3 in the government. He no doubt had ambitions on Zhao Ziyang's job but he was also a hawk that was suspicious of both the way students managed to stay in the Square day in and day out (i.e. how was it they had tents?) and the way in which Zhao was so sympathetic. He suspected Zhao was recruited by CIA and thus ordered in troops from another region (possibly Jiling, near Shanghai) to "counter-balance" troops from Beijing loyal to Zhao. Having been educated in the USSR, Li Peng was a firm believer in Soviet KGB tactics and wanted a severe retaliation against both the "students" AND their suspected supporters, i.e. Taiwan and USA.
Deng Xiaoping was in a precarious position. His no.2 and no.3 men were going at each other and recruiting other ministers to their side, effectively dividing the government. One day Li would issue stern warnings against the students while Zhao would effectively rally them the next. With both sides trying to recruit both ministers and troops to their respective factions Deng Xiaoping had to put an end to this division. He also had to seperate this troops, lest the troops under Li's and Zhao's respective influences fire upon each other and the civilians. Therefore, Deng brought in his best troops that were under his direct command, what is now known as the 15th Airborne Army. The 15th Airborne was tasked with protecting the Paramount Leader's compound (Deng Xiaoping) and to swiftly put down, by force if necessary, any troops attempting a mutiny.
Explosion
With so much tension and confusion all around, a single spark would ignite this powder keg. From late April and early May "students" were already attempting to bait PLA troops into opening fire by attacking them with rocks and bottles, but the troops wouldn't fire. However, the incident, which is NEVER shown in Western media, occurred when a "student" threw a bottle at a soldier, except this bottle was filled with gasoline and ignited, burning that soldier alive. From then on these conscript soldiers acted the only way they were trained to, which was to fight back with deadly force.
Improperly trained for such a mission, these conscript soldiers believed they were now at war and acted as such. Li's troops would target the student "leaders," the ones they felt were responsable for attacking them. Their instinct was to kill and punish those that killed their platoon mates, and one of those shameful consequences was the crushing of some students by tanks. And Zhao's troops would fire back at these troops; caught in the crossfire were many civilians and the elements of the 15th Airborne, who would fire on both troops and some dying of course.
Aftermath
Precise figures of dead or wounded are still not known to this day, with estimates ranging in the hundreds to the thousands. What was immediately known was that the country was briefly at civil war and heads had to roll.
Zhao Ziyang was considered most responsable for inflaming the situation in the first place and he was therefore dismissed as Prime Minister and placed under lifetime house arrest. Li Peng received credit for being "correct" about the plot against China and thus escaped much of the punishment. However, he was never allowed to have a high influence in government again; despite being Prime Minister, he was clearly a "lame duck" until Jiang Zemin was ready to be leader and Zhu Rongji to be Prime Minister.
Deng Xiaoping personally oversaw investigations into corruption, with many senior PLA and government officials arrested and some senior bureaucrats executed. Deng also groomed Hu Jintao to eventually become Paramount Leader and entrusted him to carry out further reforms. And, of course, Deng oversaw the specialised training for Hong Kong garrisson forces from the PLA that would be trained for non-lethal riot control. Unfortunately, Deng died in early 1997, just before he could witness this special force go into Hong Kong.
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I hope you take the time to read this and realise that the PLA, overall, does not take joy in killing its own people. And thanks for reading.