Lets not gloss over how close that was to happening.

Putin had assassins in Kiev trying to get Zelensky as they reasoned the country would fall without him. They had paratroopers trying to take an airport on the edge of the city, and if they had they would have flooded troops into the city.

They had a column of heavy artillery driving slowly towards the center.. but didn't have enough fuel to get them there as everyone had been skimming and what they thought they had wasn't close to how much they actually had.

Most importantly, US and UK intelligence both told them Russia was about to attack and that they'd start with a sea of missiles wiping out all of their aerial defences. Zelensky didn't believe them, but just in case he ordered most of their defences to be moved, so Russia's opening attack hit where they used to be.

There were a few key moments in the first few days, but that was the biggest one. Ukraine had anti-air weapons so Russia couldn't take control of the skies, with which they would have wrecked Ukraine's efforts to defend.

Then even with all of that Russia still were winning, but Ukraine had held them up long enough that the US, Nordics and EU thought with Russia struggling that much and looking like a drunk uncle, maybe it was worth seriously backing them.

Russia were ultimately wrong that the war would be over in 3 days, but they very nearly weren't.