Putin wants to reclaim the mother land of the USSR
https://nypost.com/2014/05/03/putins-vengeful-plan-to-recapture-the-old-russian-empire/
The goal with Ukraine is to capture the eastern side of Ukraine east of the river.
https://euromaidanpress.com/2015/06/17/ukraine-gets-hold-of-russian-plan-for-large-scale-invasion/
The justification is that these regions are mostly pro Russian speaking citizens
https://www.cnn.com/interactive/2014/02/world/ukraine-divided/
Putin sent an initial strike force to take 3 territories on the border of Russia. Khakiv, Luhansk, and Donetsk.
https://apps.washingtonpost.com/g/page/world/russias-buildup-on-the-ukraine-border/996/
This initial attack went well so he decided to push to the river. That push did not go well, so he is retreating back to the initial controlled territories along the border.
https://www.newsandguts.com/russia-retreats-from-kyiv-shifts-focus-to-donbas/
https://www.newstalk.com/news/russia-announces-retreat-to-eastern-ukraine-1325573
This retreat is more of a regroup. Because Putin declared war on Ukraine. The formal declaration allows Putin to deploy all the troops in reserve into Ukraine.
https://www.republicworld.com/world-news/russia-ukraine-crisis/putin-can-declare-formal-war-on-ukraine-by-may-9-for-full-mobilisation-of-forces-us-articleshow.html
So the retreat is more of a regroup and await reinforcements to then push on to the river.
I do not support this war, I am merely outlining what I see as the military strategy that Putin is using.