One would think that in problems between adversaries, red lines on which strong reaction by the side that draws them are clear and immovable. They are supposed to be a serious, risky warning that certain behavior will generate a strong reaction. In fact, red lines are quite flexible. Sometimes they move. Sometimes they are mirages that simply disappear. But always they are what is perceived to be a credible and very costly risk for those against whom the line is drawn.
It is unquestionably a dangerous game. Any red line of the moment is very difficult to openly challenge, as there is always the possibility that it could trigger a reaction even if the line is unreasonable. Yet it is unlikely that any way can be found to differentiate which red lines are for real and which are not. We have a new Cabinet office in the US -- the Department of Homeland Security -- which uses different colors to signify the degree of danger in a possible threat. It won't work with red lines. (There are some who believe it hasn't worked well with that department's terrorist threats either).
America's perception of China's red lines have not been all that accurate. Some of these red line threats disappeared. As for China, some, like firing missiles into the Taiwan Strait and verbal threats of the dire consequences of voting for the DPP both in the 2000 presidential elections and in the last Legislative Yuan elections, won it just the opposite of what it sought.
When in the early 1990s then-premier Hao Pei-tsun (郝柏村), a conservative mainlander with an impressive military resume, was relieved of his position by the president, many in Washington thought this was crossing a PRC red line. Hau was as anti-communist as one could get, but he saw Taiwan as part of (his own kind of) China, and openly said he would not defend an independent Taiwan. His successor was inevitably a Taiwanese, for the first time placing both the presidency and the premiership in local hands.
Reason enough, it was thought, that a red line would be crossed. Apparently there was no red line, or the leadership in Bei-jing couldn't decide if one had been crossed. In any event, there was a louder reaction from the then more feisty DPP opposition than there was out of Beijing.
When then president Lee Teng-hui (李登輝) first sought a change in the Constitution to allow the direct election of the president, the move was initially blocked by conservatives in his own party. They had cleverly used the lunch hour during the KMT Party Central Committee to monopolize the afternoon's speeches. Lee backed off temporarily, but eventually won the acceptance of the party, and the amendment to the Constitution was passed.
Many in Washington thought then that strengthening the legitimacy of Taiwan's president would not be tolerated by Beijing. There was no reaction then, but by the time of the next challenge, Beijing apparently had had sufficient time to learn that many countries believe the direct election of a president in Taiwan made that president more legitimate than the one in Beijing. That was included peripherally, with Lee's trip to Cornell University, as justification of some sort for lobbing missiles into the Taiwan Strait in 1995 and 1996. If there was a red line involved, the reaction turned out to be counterproductive for China.