To elaborate on this point: Russia is trying to 1 take Ukraine and 2 keep it. They have not succeeded in part 1, and part 2 is much much more difficult. Afghanistan, as stated in the above comment, is a good example of this: the war to take Afghanistan took less than a year, whereas the struggle to keep it under control was failing two decades in.

Russia can't even do the easy part.

Of course, this is assuming that they're not just trying to keep hold of Crimea so they have a port on the black sea, which I think is the real reason they're prosecuting the war.