the Russians gambled in the initial part of the war with lightning advances from the north, south and east and they were partially successful in taking large portions of Kherson and Zaporizhzhia, they encircled Mariupol and took Kherson city almost without a fight... in the north, they took land quickly but without the troops to protect the rear, the Ukrainians were successful in ambushing Russian columns and Kiev artillery did a lot of damage to the Russians... the Russians were successful in taking land around Kiev and the airport but again, 40,000 is nowhere near enough to take a sprawling city where there is strong resistance (eg. more Russian troops were involved in the fighting in Mariupol which is a city of 450,000 than Kiev which has/had 3 million)... but the Russians were looking for a negotiated settlement and they were close to it in Istanbul until Boris Johnson flew to Kiev in early April and according to the Ukrainians, scuttled any potential peace deal
from that point on, the Russians withdrew from Kiev and focused on the fighting in the Donbass where they realised they had the artillery advantage and have pursued a war of attrition... see -->
"The Russian Strategy
The Russian forces are limited by manpower but strengthened by massive artillery and equipment stockpiles enabled by a robust military industrial complex. While there have been numerous reports in Western media that the Russian army is running out of artillery ammunition, so far there’s been no visible slacking of Russian artillery fire on any front. Based on these factors, the Russian side has relied on a traditional firepower-centric war of attrition. The goal is to force an unsustainable casualty rate, destroying Ukrainian manpower and equipment, while preserving Russia’s own forces. Territory is not important; its loss is acceptable to preserve combat power. At Kyiv, Kharkiv and Kherson, the Russian army refused to fight under unfavorable conditions and withdrew, accepting the political cost to preserve its forces.
To execute this strategy, the Russian army relies on firepower, particularly its artillery. Each Russian brigade has three artillery battalions compared to just one in each Western brigade. Paired with correction by massed quantities of UAVs and quadcopters, Russian artillery pulverizes Ukrainian forces before infantry mops up survivors. It is a slow, grinding war, but with a casualty ratio that is significantly in Russia’s favor. Russia couldn’t attack because it lacked the manpower to secure the flanks of advancing troops. Up to now, Russians could only advance in Donbas, where advance did not extend the frontline. Even here the intent was more to draw in Ukrainian forces and destroy them rather than capture the city of Bakhmut. Mobilization has the potential to overcome Russia’s manpower shortages and enable offensive operations, while equipping its forces is possible due to the mobilization of industry. Precision munition production is also up, despite consistent doubt in Western press. Video of strikes by Russian "Lancet 3” loitering kamikaze drones is up up by 1,000% since Oct. 13, according to one estimate, indicating a major increase in production."
www.russiamatters.org
Russia made errors in their campaign -->
* creating a 1,000 km frontline with limited manpower and hence, the subsequent withdrawals from Kiev, Kharkov and Kherson;
* not mobilising earlier;
* not instituting a proper rotation of troops as theirs is a contract army and a lot of the troops signed 6 month contracts with the SMO commencing in February and so, when the 6 months was up, a lot returned to Russia in August-September which coincided with Ukraine's counter offensives which shows the West was providing intel to the Ukrainians and likely even planned to Kharkov operation at least;
* not building proper defences in the Kharkov region;
* a lack of a unified command with LNR, DNR, Chechen, Wagner and Russian Army all operating somewhat independently
but the Russians have solved a lot of these issues