It's coming from the fact that troops approaching Kyiv were supplied with 3 day rations and full dress uniforms, ready to hold a parade in Kyiv. It's also coming from US intelligence (https://www.washingtonpost.com/world/interactive/2022/russia...). And on top of that, russian propaganda literally repeated times and times again that this war will last for 3-4 days.
> Putin was raised in the KGB, I think he knows a bit how to get intel
Are you saying he planned for 5 years of war and russian economy being destroyed and also planned for over a million casualties for his army?
> underestimating the will of the ukrainians to hold their ground in the beginning of the war
There you go, you just said it yourself. That's the "insight".