The Gwangju Uprising failed. The end of president-for-life Park Chung-hee's dictatorship (by his assassination) revived hopes of a return to democracy. As it became apparent that a transition to democracy was not going to happen by itself, the pro-democracy movement became more active - the protests in Gwangju were part of this. The government brutally suppressed the protests, with perhaps 2000 people killed in Gwangju (thus, the more common name of "the Gwangju Massacre" rather than uprising).

One key point is that the US was intimately involved with Korea, with tens of thousands of military personnel stationed there, and an American commander in charge of the combined Korean-American force. The Gwangju Massacre was met by American acquiescence, and the US helped maintain the official government version of the Gwangju Massacre - a communist uprising was stopped.

Fast-forward to 1987, when President Chun, the military strongman who had been in charge since Gwangju, was nearing the end of his term. He suspended talks about constitutional reform (officially just postponed until after the Seoul Olympics of 1988), and named another general as his successor. The pro-democracy movement had been active (which is why there were talks about constitutional reform), with major protests (or riots, if you prefer) in Seoul about every month. With the suspension of progress towards reform, and the prospect of a further presidential term of military rule, pro-democracy action accelerated, with major protests in Seoul become weekly and even more frequency, and also large protests in other cities. As the protests grew rather than went away, the government ordered the army into the streets to suppress the protests. This would have been Gwangju multiplied by 10, just in Seoul alone. With a US general in command, news of the mobilisation order reached his ears (even if the order had bypassed the proper chain of command), and the combined armed forces commander told the US ambassador, who had words with the president: this time, the US would not accept that kind of violence. The mobilisation order was cancelled that same day, before the army was sent into the streets. The US president of the time, Reagan, who was friendly with the Korean leadership, had softer words with the Korean president, but still urged him to accept the demands of the pro-democracy movement. The Korean government accepted that their only choices were to accept a transition to democracy, or be violently overthrown. They chose the former.

Why did the Gwangju Uprising fail, and the June Democracy Movement in 1987 succeed? Gwangju, while a major city, was not Seoul, and the government felt that it could suppress the protests and stop them from spreading. Their tight control of the media meant that the official version of the event was widely accepted. US acquiescence was also important.

The June Movement in Seoul in 1987 was much bigger in scale. First, even control of the media would not have stopped news that the government had violently suppressed the pro-democracy movement (rather than a communist uprising) from spreading. Second, the US was not willing to accept such violent suppression. Without US involvement in Korea, the outcome of the June Movement in 1987 might have been similar to the 8888 Uprising.