Russia has learned a lesson from the colonial failures of the last 100 years. Russia does not try to militarily occupy and administer hostile populations. The West has portrayed this as weakness.
The Ukraine War demonstrates this. The West's oft stated goal was to weaken Russia by drawing it into an Afghanistan like situation. See this 2019 RAND study: Extending Russia, Competing from Advantageous Ground, for example. Rather than fall into this trap, Russia has made clear that its objective is not to conquer Ukraine, but rather to protect and integrate the ethnic Russian population that was engaged in civil war with the western Ukraine and which clearly wants to be part of Russia.
While Russia militarily invaded central and northern Ukraine, this was part of an effort to weaken the Ukraine military that was attacking Donbass, and was also intended to force Ukraine to the bargaining table. When the latter objective (negotiations) failed, the Russians quickly withdrew from central and northern Ukraine to focus their efforts on the areas with a supportive Russian population. They do not want to administer the hostile parts of Ukraine -- a war torn region of 25-30 million people.
Given recent experiences in countries such as Vietnam, Iraq, and Afghanistan, this seems like a prudent course of action, and one which has highlighted the West's unwillingness to learn. The conventional wisdom in the West has been that:
- Russia would quickly conquer all of Ukraine.
and - It's failure to do so was a sign of weakness.