In Feb 2022 Putin knew the US and Europe combined could not produce a significant fraction of Russia’s production of artillery systems, artillery replacement barrels and most importantly artillery ammunition. Putin knew the lead time for the West to match Russia’s artillery production, if such a decision was even taken, was best measured in decades.
Putin knew Ukraine could not ultimately mobilize half as many soldiers.
Putin knew he had the ability to neutralize Ukraine’s air force with long-range missiles. Putin knew the Patriot system can’t defend itself against hypersonic missiles, much less anything else. Putin knew the US and European stocks of SAMS of all types was limited and could be depleted by sending in cheap missiles and drones.
Putin and his military commanders have consistently shaped the battlefield to maximize Ukrainian attrition rather than territorial gains.
Putin knew all these factors would inevitably converge to this point of imminent collapse of Ukrainian military resistance.
Putin won before he started.
So he's doing war of attrition. That was not his original plan.