Upvote:3
I believe that the present system of international diplomacy began during the Italian Renaissance and spread to the rest of Europe and then to the rest of the world.
So before the Italian Renaissance diplomacy would have been much different from modern diplomacy, and I think that it would have been very unusual for third parties to make statements about whether or not they recognized the conquests and annexations that one country, state, empire, kingdom, realm, etc. made at the the expense of another.
Thus, if I understand it correctly, the time frame for your question might be restricted to the last 500 or 600 years of history.
On the other hand, whenever the Chinese Empire was mighty, the rulers of other east Asian countries and other countries in the Chinese cultural sphere, would have tributary relations with China acknowledging nominal Chinese overlordship. So it is possible that in some cases, the Chinese government would recognize and, in other cases, not recognize any conquests that one tributary state made at the expense of another one. But I don't know a lot about Chinese diplomatic relations.
Upvote:4
The Turkish occupation of Cyprus in 1974 fits your requirements. In 2024, it will be occupied successfully for half a century.
Upvote:6
It seems to me that the annexation of the Baltic States (Lithuania, Latvia, and Estonia) by the Soviet Union, from 1944 to 1991, is a good example. In particular, many countries have never accepted that that occupation was legitimate.