After the An Lushan Rebellion (755–763) was suppressed, the office of Protector General was given to Guo Xin, who defended the area and the four garrisons even after communication had been cut off from Chang’an by the Tibetan Empire. The last five years of the protectorate are regarded as an uncertain period in its history, but most sources agree that the last vestiges of the protectorate and its garrisons were defeated by Tibetan forces by 790, ending nearly 150 years of Tang influence in Central Asia.
The original OP probably used a different source, and the exact duration may have been linked to a particular estimate. But the 750s to 790s confirms that general timeline.
Context From Original OP:
Context From Wiki:
https://en.wikipedia.org/wiki/Protectorate_General_to_Pacify_the_West
I’m not seeing anything in the Wikipedia article to back up the number of 42 years. Am I missing something?
The original OP probably used a different source, and the exact duration may have been linked to a particular estimate. But the 750s to 790s confirms that general timeline.
That’s just forced resettlement with extra steps.