I have heard that good parenting advice tends to stress consistency; if you "lay down the law" you have to stick with it and not allow "infractions" to go unnoticed or unconcequenced.
For purposes of illustration of what I mean by "back down" or not:
- Some suggest that the US remained in the Vietnam War as long as it did to demonstrate (perhaps to the Soviet Union) that it was willing to continue to accept large numbers of US casualties in order to prevail and not back down.
- Obama's "red line" in Syria and the lack of apparent consequences when it was crossed1,2 may be seen as "backing down" for the purposes of this question.
One clear example of backing down - stating a "red line" or something equivalent, something that must be done by another entity, or not done, followed by an apparent lack of consequences when the line was crossed will be sufficient to confirm "has ever".
Note that for the purposes of this question the "red line" will have to be an official position articulated by the Chinese government, not something simply spoken by a member of the PLA into a microphone in the South China Sea.
Hong Kong and Macau are China's SAR's, the Tibet Autonomous Region is TAR, and for the purposes of this question, cross-straits interactions with Taiwan will count as well.
1The Atlantic, June 3, 2018 Inside the White House During the Syrian 'Red Line' Crisis "We in the Obama administration stepped up to the brink of military action against Assad. And then, suddenly, we stepped back."
2Politico, July 19, 2016 Obama’s Red Line, Revisited "The offhand remark spurred a massive success in Syria. Why does the foreign policy establishment consider it a failure?"