It is indeed an annexation. But it is also true that the majority of the inhabitants of Crimea are Russians and wanted to return to join Russia.
But all the consequences, not the cause. The reason, in my opinion, is the global conflict between the West and Russia. And, obviously, the fall of Russia is just a matter of time.
Despite the fact that they have weapons, in everyday life, all residents use technologies and products made in the west. Everyone has an iPhone, Laptop, Internet, car, etc. How can you win a conflict if every inhabitant of your country is completely dependent on technology, software, products and money created by a conditional "enemy"?