Unfortunately totally legal under international law with the United Nations blessing…
In 1972, China successfully petitioned the United Nations to remove Hong Kong from its list of non-self-governing territories, and declared that the colony was a “Chinese territory under British administration”. The United Kingdom did not raise any objections to this and the local population did not think the move was significant, but the implication of this change was that Communist China alone would determine the territory’s future, excluding the people of Hong Kong.
https://en.m.wikipedia.org/wiki/Sino-British_Joint_Declaration
This would probably rank amongst the more verifiable things…